Membrane permeation treatment with adjustment of the temperature of the first retentate as a function of the ch4 concentration in the third and/or fourth permeate

The method for the membrane permeation treatment of a feed gas flow containing at least methane and carbon dioxide, comprising the steps of:

• compressing the feed gas flow with a compressor;
• receiving a flow of gas coming from the compressor at a first membrane separation unit that supplies a first permeate and a first retentate;
• receiving the first retentate at a second membrane separation unit that supplies a second permeate and a second retentate;
• receiving the first permeate at a third membrane separation unit that supplies a third permeate and a third retentate;
• receiving the third retentate at a fourth membrane separation unit that supplies a fourth permeate and a fourth retentate;
• with a gas analyser, measuring a CH4 concentration of the third permeate, a CH4 concentration of the fourth permeate, or a CH4 concentration of a flow of gas that is comprised of a combination of the third permeate and the fourth permeate;
• comparing the measured CH4 concentration with a setpoint CH4 concentration and determining a difference therebetween;
• and
• adjusting a temperature of the first retentate at an inlet of the second membrane separation unit with at least one heat exchanger as a function of the measured CH4 concentration in such a way so as to reduce the determined difference, wherein each membrane separation unit comprises at least one membrane that is more permeable to carbon dioxide than to methane.