Method for purifying biogas through membranes at negative temperatures

Process for purifying a gas stream comprising methane and carbon dioxide by membrane permeation, said process comprising the following successive steps:

• a) compressing the gas stream to a pressure between 5 and 20 bar;
• b) a first step of cooling the compressed gas stream to a temperature between 0° C. and 15° C.;
• c) drying the cooled and compressed gas stream to obtain a water content ≦0.1 ppm;
• d) a second step of cooling the gas stream resulting from step c) using a heat exchanger to a temperature between 0° C. and −60° C.;
• e) separating the gas stream resulting from step d) through at least one membrane stage so as to obtain a CO2-enriched permeate and a CO2-depleted retentate;
• and
• f) recovering a methane-enriched gas stream.

A plant for purifying a gas stream comprising methane and carbon dioxide by membrane permeation, said plant comprising, in the flow direction of the gas stream:

• a) a compressor configured to compress the gas stream to between 5 and 20 bar,
• b) a cooling means configured to cool the gas stream to a temperature between 0° C. and 15° C.,
• c) a dryer configured to dry the cooled and compressed gas stream so as to obtain a gas stream having a water content of less than 0.1 ppm,
• d) an exchanger configured to cool the gas stream to a temperature between 0° C. and −60° C.,
• e) a separation unit comprising at least one membrane stage more permeable to carbon dioxide configured to separate the gas stream leaving the exchanger.