Ammonia synthesis takes place on an iron catalyst at a pressure of 100-250 bar and temperature of 350-550^oC according to the following reaction (IPTS/EC, 2007 p.43):

N₂ + 3H₂ ↔ 2NH₃,   ΔΗ_ο = - 46 kJ/mol

Only about 20-30% of the synthesis gas is converted to ammonia due to unfavorable equilibrium conditions. The ammonia produced is separated by cooling/condensation. After the ammonia formed is removed, the unreacted gas is recycled back to the converter and the reacted gas is compensated with new make-up synthesis gas.

As the exothermic reaction in the ammonia synthesis reactor proceeds with a decrease in volume, lower temperatures and higher pressures will favor the reaction. Controlling the catalyst temperature is important as the heat of reaction raises the temperature. One technique for temperature control is the subdivision of the catalyst into several layers. With this method, the temperature of the gases can decrease by adding cooled synthesis gas between the layers or indirectly by generating steam (IPTS/EC, 2007 p.43).   

Steam reforming with methanation as the final purification step results in a synthesis gas with high inerts content (unreacted methane and argon). The inert contents are controlled by taking out a purge gas stream from the loop. The size of the purge gas stream can regulate the inerts content to about 10-15%. The purge gas is scrubbed with water in order to remove the contained ammonia and then sent for use as a fuel in the primary reformer or for hydrogen recovery (IPTS/EC, 2007 p.43).

Compression

The synthesis gas needs to be compressed to high pressures, ranging from 100 to 250 bars, for ammonia synthesis. Modern plants employ centrifugal compressors which are usually driven by steam turbines that use the steam produced from excess process heat (IPTS/EC, 2007 p. 42). The energy requirements in a modern steam reforming plant are 40-50% above the thermodynamic minimum. Compression losses are responsible for more than half of the excess energy use (EFMA, 2000 p. 7).

The steam requirements in the turbines used for driving the synthesis gas compressor, the air compressors and the refrigeration compressors are about 3.9-6.5 GJ/t NH₃ (Ecofys, 2013). With the use of more active ammonia synthesis catalysts, such as cobalt-promoted iron and ruthenium, lower pressures can be employed and the energy use can decrease (IPTS/EC, 2007 p.43).