Liquid Nitrogen Wash
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® scrubbing process, if partial oxidation of coal or residue oil were selected as feedstock basis for ammonia production. The unit is generally supplied as assembled coldbox and includes an upstream adsorber station.
Example
Customer:
Dalian Chemical Industrial Corp.
Plant location:
Dalian / P.R. China
Process:
Liquid nitrogen wash for the removal of CO, Ar and CH4 down-stream of the RECTISOL wash
Capacity:
85,000 Nm3/feed gas
equivalent to 1,000 t/d of ammonia
Scope of work:
General contract for turn-key project, process, basic and detail engineering, manufacturing of equipment, supervision of commissioning and start-up
Liquid Nitrogen Wash
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 purgegas separation are required.
Feed gases for liquid nitrogen wash units are frequently produced by partial oxidation processes with residue oil, pitch or coal as feed.
The liquid nitrogen wash consists of
an adsorber unit to remove traces of water, CO2 and solvent (methanol) to prevent freezing in the low temperature section and
the actual liquid nitrogen cold box, where all the cryogenic process equipment is assembled for supply as package unit.
Basic Flow Diagram: Liquid Nitrogen Wash
Raw hydrogen and HP 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 partially condensed nitrogen to the top, trace components are removed and separated as fuel gas. To establish the desired H2/N2 ratio, HP nitrogen is additionally admixing inside and outside the coldbox. Manufacturing of cryogenic equipment and assembly as transportable coldbox is carried out in Linde workshops.
References:
Rainer Zeller, Liquid Nitrogen Wash, Handout brochure to be received through Linde
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