Gas separation system and method for producing methane-enriched gas

A gas separation system for enriching methane contained in raw material mixed gas comprising at least carbon dioxide and methane by supplying the raw material mixed gas to gas separation membrane unit, wherein:

• the gas separation system comprises a first gas separation membrane unit and a second gas separation membrane unit;
• each of the gas separation membrane units comprise at least a gas inlet, a permeated gas outlet, and a non-permeated gas outlet;
• the raw material mixed gas contains 30 mol % or more of CH4 and 3 to 70 mol % of CO2,
• 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 non-permeated gas discharge line;
• the raw material mixed gas supply line is connected to the gas inlet of the first gas separation membrane unit, a compressor is interposed in the raw material mixed gas supply line, and a first permeated gas recycle line connects the permeated gas outlet of the first gas separation membrane unit and a position on a suction side of the compressor on the raw material mixed gas supply line;
• the first permeated gas recycle line comprises a permeated gas discharge line for at least partially discharging the permeated gas discharged from the first gas separation membrane unit to the outside of the gas separation system, and no gas separation membrane is interposed in the first permeated gas recycle line;
• the permeated gas outlet of the second gas separation membrane unit and a position on the suction side of the compressor in the raw material mixed gas supply line are connected by a second permeated gas recycle line;
• enriched methane is recovered from the non-permeated gas outlet of the second gas separation membrane unit, and
• a ratio of a recycle flow rate F4 circulated to the first gas separation membrane unit by the first permeated gas recycle line to a permeated gas flow rate F1 of the first gas separation membrane unit is 0.5% or more and 60% or less.