Abstract: A gas separation system for enriching methane contained in raw material mixed gas including at least carbon dioxide and methane by supplying the raw material mixed gas to a gas separation membrane unit in which the gas separation system includes a first gas separation membrane unit and a second gas separation membrane unit. A raw material mixed gas supply line is connected to a gas inlet of the first gas separation membrane unit. The non-permeated gas outlet of the first gas separation membrane unit and the gas inlet of the second gas separation membrane unit are connected by a first non-permeated gas discharge line. A second non-permeated gas recovery line is connected to the second non-permeated gas outlet of the second gas separation membrane unit.

Abstract: A raw material mixed gas supply line 13 is connected to a gas inlet of the first gas separation membrane unit 11. The non-permeated gas discharge port 11b of the first gas separation membrane unit 11 and the gas inlet of the second gas separation membrane unit 12 are connected by a first non-permeated gas discharge line 14. A second non-permeated gas recovery line 15 is connected to the second non-permeated gas discharge port 12b of the second gas separation membrane unit 12. The first permeated gas discharge port 11c of the first gas separation membrane unit 11 and the second permeated gas discharge port 12c of the second gas separation membrane unit 12 are connected to the raw material mixed gas supply line 13 by a first permeated gas recycle line 16 and a second permeated gas recycle line 17, respectively. The line 16 includes a permeated gas discharge line 18 for at least partially discharging the permeated gas of the first gas separation membrane unit to the outside of the system.