US-12623983-B2 - Gas separation system and method for producing methane-enriched gas

US12623983B2US 12623983 B2US12623983 B2US 12623983B2US-12623983-B2

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. 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.

Inventors

Nobuhiko Fukuda
HIROKI INDE
Takumi Fukunaga
Tomohide Nakamura

Assignees

UBE CORPORATION

Dates

Publication Date: 20260512
Application Date: 20230712
Priority Date: 20220714

Claims (13)

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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application is the U.S. National Phase under 35 U.S.C. § 371 of International Application PCT/JP2023/025794, filed Jul. 12, 2023, designating the U.S., and published in Japanese as WO 2024/014493 on Jan. 18, 2024 which claims priority to Japanese Patent Application No. 2022-113548 filed Jul. 14, 2022, the entire content of which is incorporated herein by reference. TECHNICAL FIELD The present invention relates to a gas separation system for separating a mixed gas using a plurality of gas separation membrane units, and a method for producing a methane-enriched gas using the same. BACKGROUND ART As a method of separating a mixed gas containing two or more kinds of different gases into respective gases, a membrane separation method using a difference in permeation rate of a gas with respect to a membrane is known. In this method, a high-purity high-permeability gas and/or a high-purity low-permeability gas, which are target gases, can be obtained by collecting the permeated gas and/or the non-permeated gas. The permeation rate, which is the permeation volume per unit membrane area, unit time, and unit partial pressure difference of each gas contained in the mixed gas with respect to the membrane, can be expressed by P′ (unit: ×10−5 cm3 (STP)/cm2·sec. cmHg). The gas separation selectivity of the membrane can be expressed as a ratio of (permeation rate of high permeability gas/permeation rate of low permeability gas). In general, in a gas separation membrane, a membrane having a high gas separation selectivity has a low gas permeation rate, whereas a membrane having a high gas permeation rate has a low gas separation selectivity. Therefore, in the case where the low-permeability gas is recovered from the mixed gas using the single-stage gas separation membrane, when the purity of the recovered gas is constant, the recovery rate becomes high when the membrane having high gas separation selectivity is used. However, due to the low permeation rate, it is necessary to increase the membrane area or to increase the operating pressure. On the other hand, in the case of a membrane having a high permeation rate, it is not necessary to increase the membrane area or the operating pressure, but the recovery rate is low because of the low gas separation selectivity. In general, a gas separation membrane having selective gas permeability is used as a gas separation membrane module in which the gas separation membrane is accommodated in a container provided with at least a gas inlet, a permeated gas outlet, and a non-permeated gas outlet. The gas separation membrane is mounted in the container such that a space on the gas supply side and a space on the gas permeation side are isolated from each other. In a gas separation system, a gas separation membrane unit in which a plurality of gas separation membrane modules are combined in parallel is generally used in order to obtain a required membrane area. Since the plurality of gas separation membrane modules constituting the gas separation membrane unit share a gas inlet, a non-permeated gas outlet, and a permeated gas outlet, the gas separation membrane unit acts as a substantially large gas separation membrane module. In order to recover a target low-permeability gas at a high purity and a high recovery rate, a method of using a system provided with this gas separation membrane unit in multiple stages is known. Examples of the multistage gas separation system include a system that further separates the first-stage non-permeated gas enriched with the low-permeability gas in order to improve purity, and a system that recovers the low-permeability gas contained in the first-stage permeated gas in order to improve recovery rate. PTL 1 is configured to: pressurize, by a compressor, a mixed gas containing an easily permeable gas that easily permeates a gas separation membrane and a hardly permeable gas that hardly permeates the gas separation membrane to supply the mixed gas to a first membrane separation unit provided with a first gas separation membrane; taking out a first permeated gas that permeated the first gas separation membrane; and supply a first residual gas that does not permeate the gas separation membrane and is discharged from the first membrane separation unit to a second membrane separation unit provided with a second gas separation membrane, a gas purification method of circulating and supplying a second permeated gas permeated through the second gas separation membrane to the raw material gas to perform gas purification, wherein a part of the first permeated gas obtained by passing through the first gas separation membrane is circulated and supplied to the raw material gas side, the mixed gas and the circulated and supplied first permeated gas are purified by the first gas separation membrane, and the remainder of the obtained first permeated gas is recovered in performing gas purification. In the PTL 1, since the purity