Process for cryogenic separation of a feed stream containing methane and air gases, facility for producing biomethane by purification of biogases derived from non-hazardous waste storage facilities (NHWSF) implementing the process

A cryogenic separation process for a feed stream containing methane and air gases comprising nitrogen and oxygen, comprising:

• cooling the feed stream to produce a cooled stream comprising methane, nitrogen, and oxygen;
• at least partially condensing the cooled stream by heat exchange;
• decompressing the at least partially condensed cooled stream to produce a decompressed stream comprising a gaseous phase and a liquid phase, wherein the gaseous phase consists essentially of N2;
• separating the gaseous phase from the liquid phase;
• injecting the liquid phase into a distillation column at a first level;
• adding the gaseous phase into the distillation column at a second level;
• drawing a bottom stream off of the distillation column, where the bottom stream is enriched in methane compared to the feed stream;
• and
• drawing off a stream which is enriched in oxygen and nitrogen compared to the feed stream off of the distillation column, wherein:
• the distillation column comprises two or more distillation segments, the gaseous phase that consists essentially of N2 is incombustible and more volatile than oxygen, and is added into the distillation column,
• the cooled stream is at least partially condensed by heat exchange with a fraction of the bottom stream which further cools the cooled stream resulting in the cooled stream being at least partially condensed and the fraction of the bottom stream being boiled to produce a methane-rich gas which is then added into the distillation column at a third level,
• the second level, at which the gaseous phase is added to the distillation column, is below the first level, at which the liquid phase is injected into the distillation column, and above the third level, at which the methane rich gas is added into the distillation column,
• at least one of the two or more distillation segments is interposed between the second and third levels, and
• the decompressed stream is mixed with a refrigerant fluid before the gaseous phase and the liquid phase are separated.