Treatment of purge gas from ammonia synthesis - to recover and recycle hydrogen

摘要

The treatment of a purge gas from an ammonia synthesis plant, contg. a large proportion of hydrogen and nitrogen and non-negligible amts. of argon, methane, ammonia and helium, involves a preliminary extn. of ammonia. The gas is then fractionated by partial condensn. in an exchanger, with formation at the cold end of a H2-enriched vapour fraction and a liq. fraction enriched in argon and methane. At least a large part of the H2-enriched gaseous fraction is recycled into the synthesis cycle. The liq. fraction is expanded to a vaporisation pressure by heat exchange at counter-current with the purge gas during cooling. The vaporised liq. fraction is, during the heat exchange, expanded to a second low pressure in an expansion turbine with external work, before being reheated by thermal exchange at counter-current with the purge gas.