The production of ammonia requires the adjustment of the stoichiometric nitrogen/hydrogen ratio to a value of 1:3.
Simultaneously other trace impurities like methane, argon and carbon monoxide (below 5ppm) should be removed to preferentially operate the ammonia synthesis in the absence of inert components. The liquid nitrogen wash is typically arranged downstream of the RECTISOL® srubbing process, if partial oxidation of coal or residue oil were selected as feedstock basis for ammonia production. The unit is generally supplied as an assembled coldbox and includes an upstream dryer unit.
Traces of water, carbon dioxide, solvent (methanol) are removed in the adsorber station. The process equipment of the cryogenic separation is installed in the coldbox, which is covered with a metal shell. The coldbox voidage is filled with insulation material (Perlite) to prevent heat input. The coldbox is typically completely prefabricated and delivered in one piece on site.
The Liquid Nitrogen Wash is mainly used to purify and prepare ammonia synthesis gas within fertilizer plants. It is usually the last purification step upstream of ammonia synthesis.
The liquid nitrogen wash has the function to remove residual impurities like CO, Ar and CH4 from a crude hydrogen stream and to establish a stoichiometric ratio H2 / N2 = 3:1. Carbon monoxide must be completely removed, since it is poisonous for the ammonia synthesis catalyst. Ar and CH4 are inert components enriching in the ammonia synthesis loop. If not removed, a syngas purge or expenditures for purge gas separation are required.
Raw hydrogen and high pressure nitrogen are fed to the liquid nitrogen wash unit. Both streams are cooled down against product gas. Feeding raw hydrogen to the bottom of the nitrogen wash column and some condensed nitrogen liquid to the top. Trace components are removed and separated as fuel gas. To establish the desired H2/N2 ratio, high pressure nitrogen is added to the process stream.
Manufacturing of cryogenic equipment and assembly as transportable coldbox is carried out in Linde workshops.