Plant and process for ammonia production with cryogenic purification, and related method of revamping

A process for the synthesis of ammonia from a make-up gas containing hydrogen and nitrogen, said process comprising the steps of:

• production of a synthesis gas containing hydrogen and nitrogen in a molar ratio lower than 3, inside a front-end section;
• a first cryogenic purification of said synthesis gas, suitable to remove nitrogen and raise the hydrogen/nitrogen molar ratio of said gas to a predetermined value;
• compression of the purified synthesis gas to a synthesis pressure;
• conversion of synthesis gas into ammonia inside a synthesis loop, with extraction from said loop of a purge stream containing hydrogen and inert gases;
• and
• performing a second purification of at least one portion of said purge stream, by means of a process suitable to recover at least part of the hydrogen contained therein, with the production of at least one stream containing recovered hydrogen and of at least one waste stream,
• recycling said at least one stream containing recovered hydrogen to said process, in order to increase the ammonia production,
• wherein at least a part of said at least one stream containing recovered hydrogen is recycled by means of joining with the gas stream undergoing said first cryogenic purification step.