Facility and method for the treatment by membrane permeation of a gas stream with adjustment of the third-permeate suctioning pressure

1. A facility for membrane permeation treatment of a feed gas stream containing at least methane and carbon dioxide, comprising:

a compressor A for compressing the feed gas stream;

a first membrane separation unit able to receive the feed gas stream coming from the compressor A and to supply a first permeate and a first retentate;

a second membrane separation unit able to receive the first retentate and to supply a second permeate and a second retentate;

a third membrane separation unit able to receive the first permeate and to supply a third permeate and a third retentate;

at least one means for measuring a pressure of the feed gas stream at an inlet of the first membrane separation unit;

at least one means for measuring a CH 4 concentration in the second retentate; and

at least one compressor B for adjusting a third-permeate suctioning pressure as a function of a measured pressure of the feed gas stream and of a measured methane concentration of the second retentate, wherein each membrane separation unit comprises at least one membrane that is more permeable to carbon dioxide than to methane.

2. The facility of claim 1 , wherein the third retentate is recycled to the feed gas stream upstream of the compressor A.

3. The facility of claim 1 , wherein the second permeate is recycled to the feed gas stream upstream of the compressor A.

4. The facility of claim 1 , wherein the membranes used in the membrane separation units have a same selectivity.

5. The facility of claim 1 , wherein at least one of said first, second, and third membrane separation units comprises at least two membranes.

6. The method for controlling the facility recited in claim 1 , comprising the steps of:

a) measuring a pressure of the feed gas stream at the inlet of the first membrane separation unit using the at least one means for measuring a pressure, wherein the at least one means for measuring a pressure comprises at least one pressure sensor;

b) measuring a CH 4 concentration in the second retentate using the at least one means for measuring a CH 4 concentration, wherein the at least one means for measuring a CH 4 concentration comprises at least one CH 4 analyzer;

c) comparing the measured pressure of the feed gas stream and the measured CH 4 concentration in the second retentate with associated setpoint values and determining the differences between the measured pressure of the feed gas stream and the measured CH 4 concentration in the second retentate and the associated setpoint values; and

d) adjusting the third-permeate suctioning pressure of the compressor B.

7. The method of claim 6 , wherein the third-permeate suctioning pressure is adjusted by the compressor B and the pressure of the feed gas stream is adjusted by the compressor A or by a progressive shut-off and pressurizing valve.

8. The method of claim 7 , wherein the adjusting of the pressure of the feed gas stream comprises increasing or decreasing the pressure.

9. The method of claim 6 , wherein, in said step of adjusting, the compressor B is subjected to an acceleration or deceleration.

10. The method of claim 6 , wherein the comparing, determining, and adjusting steps are performed automatically by a data transmission and data processing means.

11. The method of claim 6 , wherein the feed gas stream is biogas.