Method for separating a synthesis gas

A method for separating a synthesis gas containing carbon monoxide and hydrogen comprising:

• i) compressing a flow of synthesis gas received from a source of synthesis gas in a compressor,
• ii) purifying the compressed synthesis gas in a purification unit to purify it of water and/or carbon dioxide,
• iii) cooling the compressed and purified flow of synthesis gas,
• iv) separating the cooled flow of synthesis gas by washing and/or distillation at a cryogenic temperature and optionally by adsorption in a separating unit, and
• v) producing at least the following three gases in the separating unit:
• a carbon monoxide-enriched gas, a hydrogen-enriched gas, a residual gas containing carbon monoxide and hydrogen that is less pure with respect to carbon monoxide than the carbon monoxide-enriched gas and less pure with respect to hydrogen than the hydrogen-enriched gas and optionally also a methane-enriched gas, and/or a nitrogen-enriched gas, and
• vi) sending at least one part of each of the following gases downstream of the source, only if the flow of synthesis gas received from the source and sent to the compressor is below a threshold or zero, the carbon monoxide-enriched gas, the hydrogen-enriched gas, and the residual gas containing carbon monoxide and hydrogen, to be purified in the purification unit and separated in the separating unit, wherein the at least one part of the three gases is sent upstream of the compressor to be compressed in the compressor upstream of the purification.