Gas separation system and method for producing methane-enriched gas
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. The first permeated gas outlet of the first gas separation membrane unit and the second permeated gas outlet of the second gas separation membrane unit are connected to the raw material mixed gas supply line by a first permeated gas recycle line and a second permeated gas recycle line, respectively. The line includes a permeated gas discharge line for at least partially discharging the permeated gas of the first gas separation membrane unit to the outside of the system.
1 . 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 CH 4 and 3 to 70 mol % of CO 2 ,
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.
2 . The gas separation system of claim 1 , wherein the operating temperature of the first gas separation membrane unit is different from that of the second gas separation membrane unit.
3 . The gas separation system of claim 2 , wherein the operating temperature of the second gas separation membrane unit is higher than that of the first gas separation membrane unit.
4 . The gas separation system of claim 3 , wherein a heater is disposed in the middle of the non-permeated gas discharge line connecting the non-permeated gas outlet of the first gas separation membrane unit and the gas inlet of the second gas separation membrane unit.
5 . The gas separation system of claim 1 , wherein the gas separation selectivity of the first gas separation membrane unit is different from that of the second gas separation membrane unit.
6 . The gas separation system of claim 5 , wherein gas separation selectivity of the second gas separation membrane unit is lower than that of the first gas separation membrane unit.
7 . The gas separation system of claim 1 , wherein the raw material mixed gas contains 40 to 95 mol % of CH 4 and 5 to 60 mol % of CO 2 .
8 . The gas separation system of claim 1 , wherein a pressure of the compressor is 0.2 MPaG or more and 3.0 MPaG or less.
9 . The gas separation system of claim 1 , wherein a ratio of a flow rate F2 of the permeated gas discharged through the first permeated gas discharge line to the permeated gas flow rate F1 of the first gas separation membrane unit is 30% or more and 99% or less.
10 . The gas separation system of claim 3 , wherein a difference between the operating temperature of the second gas separation membrane unit and the operating temperature of the first gas separation membrane unit is 5° C. or more and 40° C. or less.
11 . The gas separation system of claim 1 , wherein gas separation membranes in the first and second gas separation membrane units are hollow fiber gas separation membranes made of an aromatic polyimide having an asymmetric structure in which a thickness of a homogeneous layer is 10 nm or more and 200 nm or less, and a thickness of a porous layer is 20 μm or more and 200 μm or less.
12 . The gas separation system of claim 6 , wherein when a gas separation selectivity (CO 2 /CH 4 ) of the gas separation membrane of the first unit at 40° C. is normalized to 1.0, a gas separation selectivity of the gas separation membrane in the second gas separation membrane unit is 0.1 or more and 0.9 or less.
13 . The gas separation system of claim 1 , wherein a methane permeation rate P′ CH4 1 of the gas separation membrane in the first gas separation membrane unit and a methane permeation rate P′ CH4 2 of the gas separation membrane in the second gas separation membrane unit are each 0.03×10 −5 cm3 (STP)/cm2·sec·cmHg or more and 3×10 −5 cm3 (STP)/cm2·sec·cmHg or less at 40° C.