Application of liquid nitrogen washing process: the feed gas of synthetic ammonia contains nitrogen, as well as impurities like carbon and oxygen compounds (such as carbon monoxide, carbon dioxide, etc), argon and sulfides, which could not satisfy the requirements of subsequent process.  Normally rectisol plus liquid nitrogen washing process will be used to remove these impurities.  Liquid nitrogen washing process is a cryogenic purification method, using cryogenic distillation to obtain high purity hydrogen, then mix hydrogen and nitrogen by 3:1 to be used for ammonia synthesizing.  Particularly the output from cryogenic liquid nitrogen wash device has carbon monoxide content less than 2 ppm, which effectively improves the efficiency of the synthetic ammonia catalyst, reduces the synthesis pressure, eliminate the relief gas, so as to achieve the effect of energy saving and long period operation, low emissions.