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 5 ppmv) 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, heavy oil, asphalt 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.
Traces of water, carbon dioxide, solvent (methanol) are removed in the adsorber station. Centerpiece of the liquid nitrogen wash is the so called “coldbox”. The process equipment of the cryogenic separation is installed close-packed 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 generally completely prefabricated and delivered in one piece on site.
Basic Flow Diagram: Process Scheme 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 purge gas separation are required.
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 beforehand condensed liquid 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.
Example:
Liquid Nitrogen Wash in Dalian, P. R. China
Customer:
Dalian Chemical Industrial Corp.
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.
