The make-up synthesis gas contains water and CO₂ that are poisonous to the ammonia synthesis catalyst and need to be completely removed. As water and CO₂ can be fully absorbed by ammonia condensation, in older plants, the make-up synthesis gas is usually fed at the exit stream gas of the synthesis reactor. In this plant configuration (part B in Figure below) the condensation unit is placed between the compressor and the synthesis reactor. Main drawbacks are the decreased ammonia concentration (due to the dilution with make-up gas) and the higher amount of ammonia entering the synthesis reactor (at equal condensing temperatures there is higher ammonia concentration at the entrance of the converter). Additional drawback is that a larger stream gas needs to be compressed, the ammonia exiting the reactor and the make-up gas.

With the use of molecular sieve adsorbers serving as dryers, the make-up gas stream can be completely freed of water and carbon oxides prior to its entrance in the ammonia synthesis reactor.

The make-up gas can then be directly fed to the synthesis reactor (see part A in Figure below). This arrangement is the most energetically favorable (Ullmann’s, 2011 p.190). The conversion rate is improved due to the lower ammonia content in the gas entering the reactor. The improved conversion will reduce the energy requirements for recycling non-activated gas and potentially reduce the loop operating pressure and thus, reduce the power requirements for compression. The molecular sieve drying of make-up gas due to its higher energy efficiency performance, has found wide implementation, also as a revamp in existing ammonia plants (Ullmann’s, 2011 p.191).