Method for purifying natural gas using an economizer

A process for purifying a gaseous feed stream of natural gas comprising methane, CO2 and hydrocarbons containing at least two carbon atoms, comprising the following steps:

• Step a):
• cooling the gaseous feed stream in a heat exchanger;
• Step b):
• introducing stream obtained from step a) into a phase-separating chamber to produce a liquid stream depleted in methane and enriched in hydrocarbons containing more than three carbon atoms and a gaseous stream;
• Step c):
• separating the gaseous stream obtained from step b) in a first membrane permeation unit including at least one main membrane separation stage from which emerges at least one CO2-enriched gaseous first permeate stream and a partially condensed first residual stream depleted in CO2 and enriched in methane;
• Step d):
• introducing the first residual stream obtained from step c) into a phase-separating chamber to produce at least two phases, a liquid stream including at least 0.5 mol % of hydrocarbons containing at least three carbon atoms initially contained in the feed stream, and a gaseous stream;
• Step e):
• heating the gaseous stream obtained from step d) by introducing it into the heat exchanger used in step a) counter-currentwise relative to the feed stream so as to produce a gaseous stream depleted in CO2 and enriched in methane relative to the feed stream;
• and
• Step f):
• introducing the gaseous stream obtained from step e) into a second membrane separation unit, from which emerges at least one gaseous second permeate stream enriched in CO2 and a gaseous second residual stream depleted in CO2 and enriched in methane.