CN-121985989-A - Biological LNG production using membranes

Abstract

Disclosed herein are methods and systems for producing a liquefied methane product from a feed stream containing methane and carbon dioxide, wherein carbon dioxide is removed from the feed using a plurality of membrane stages including a gas separation membrane that is more permeable to carbon dioxide than to methane and a methane-enriched retentate stream is formed, which is then cooled and liquefied to provide the liquefied methane product. In particular, the disclosed methods and systems may be used to produce liquefied biomethane from biogas feeds.

Inventors

• M. P. O'Brien

Assignees

• 气体产品与化学公司

Dates

Publication Date

20260505

Application Date

20241030

Priority Date

20231101

Claims (20)

1. 1. A method of producing a liquefied methane product from a feed comprising methane and carbon dioxide, the method comprising: (a) Introducing a feed stream comprising methane and carbon dioxide into a first membrane stage comprising a first gas separation membrane that is more permeable to carbon dioxide than to methane; (b) Separating the feed stream in the first membrane stage into a methane-enriched first retentate stream and a carbon dioxide-enriched first permeate stream; (c) Introducing the first retentate stream into a second membrane stage comprising a second gas separation membrane that is more permeable to carbon dioxide than to methane; (d) Separating said first retentate stream in said second membrane stage into a second retentate stream further enriched in methane and a second permeate stream, and (E) Cooling the second retentate stream to form a liquefied methane product stream from at least a portion of the second retentate stream; Wherein the second membrane stage comprises a plurality of gas separation modules arranged in series, each of the gas separation modules comprising a housing, a bundle of hollow fibers contained within the housing, and a housing space outside the hollow fibers and inside the housing, each of the hollow fibers comprising a hole having an open end and a side wall formed of a membrane material that is more permeable to carbon dioxide than to methane, the hollow fibers together constituting all or a portion of the second gas separation membrane, and Wherein the plurality of gas separation modules are configured such that retentate gas and permeate gas flow through the gas separation modules in a countercurrent direction, retentate gas exiting from the open ends of the hollow fibers of a gas separation module being mixed prior to being introduced into the open ends of the hollow fibers of a next gas separation module in a series, wherein the retentate gas exiting the open ends of the hollow fibers of a last gas separation module in a series forms the second retentate stream, and permeate gas exiting the housing space of a gas separation module is introduced into the housing space of a previous gas separation module in a series as purge gas, wherein the permeate gas exiting the housing space of a first gas separation module in a series forms the second permeate stream.
2. 2. The method of claim 1, wherein the plurality of gas separation modules comprises at least three gas separation modules arranged in series.
3. 3. The method of claim 1, wherein the feed stream is a biogas feed stream and the liquefied methane product stream is a liquefied biomethane product stream.
4. 4. The process of claim 1, wherein the second retentate stream comprises 0.005% or less by volume carbon dioxide.
5. 5. The process of claim 1, wherein the second retentate stream has a volumetric concentration of carbon dioxide that is reduced by 1/2000 or less relative to the volumetric concentration of carbon dioxide in the first retentate stream.
6. 6. The process of claim 1, wherein step (e) comprises cooling the second retentate stream to form an at least partially liquefied second retentate stream, and flashing and separating the at least partially liquefied second retentate stream to form a flash gas and the liquefied methane product stream.
7. 7. The method of claim 6, wherein the method further comprises using a flash gas stream as a purge gas in the second membrane stage, the flash gas stream being introduced as a purge gas into the shell space of the last gas separation module in series.
8. 8. The method of claim 1, wherein the method further comprises: (f) The second permeate stream is recycled into the feed stream.
9. 9. The method of claim 1, wherein the method further comprises: (g) Introducing the first permeate stream into a third membrane stage comprising a third gas separation membrane that is more permeable to carbon dioxide than to methane, and (H) Separating the first permeate stream in the third membrane stage into a third permeate stream further enriched in carbon dioxide and a third retentate stream.
10. 10. The method of claim 9, wherein the method further comprises: (i) Said third retentate stream is recycled into said feed stream.
11. 11. The process of claim 1, wherein the process further comprises compressing the feed stream prior to introducing the feed stream into the first membrane stage in step (a).
12. 12. A system for producing a liquefied methane product from a feed comprising methane and carbon dioxide, the system comprising: A first membrane stage comprising a first gas separation membrane that is more permeable to carbon dioxide than methane, the first membrane stage configured to receive a feed stream comprising methane and carbon dioxide and separate the feed stream into a methane-enriched first retentate stream and a carbon dioxide-enriched first permeate stream; A second membrane stage comprising a second gas separation membrane that is more permeable to carbon dioxide than to methane, the second membrane stage configured to receive the first retentate stream and separate the stream into a second retentate stream that is further enriched in methane and a second permeate stream, and A heat exchanger for cooling the second retentate stream to form a liquefied methane product stream or an at least partially liquefied second retentate stream from which a liquefied methane product stream can be produced; Wherein the second membrane stage comprises a plurality of gas separation modules arranged in series, each of the gas separation modules comprising a housing, a bundle of hollow fibers contained within the housing, and a housing space outside the hollow fibers and inside the housing, each of the hollow fibers comprising a hole having an open end and a side wall formed of a membrane material that is more permeable to carbon dioxide than to methane, the hollow fibers together constituting all or a portion of the second gas separation membrane, and Wherein the plurality of gas separation modules are configured such that retentate gas and permeate gas flow through the gas separation modules in a countercurrent direction, retentate gas exiting from the open ends of the hollow fibers of a gas separation module being mixed prior to being introduced into the open ends of the hollow fibers of a next gas separation module in a series, wherein the retentate gas exiting the open ends of the hollow fibers of a last gas separation module in a series forms the second retentate stream, and permeate gas exiting the housing space of a gas separation module is introduced into the housing space of a previous gas separation module in a series as purge gas, wherein the permeate gas exiting the housing space of a first gas separation module in a series forms the second permeate stream.
13. 13. The system of claim 12, wherein the plurality of gas separation modules comprises at least three gas separation modules arranged in series.
14. 14. The system of claim 12, wherein the second membrane stage has a surface area of the second gas separation membrane configured to provide a second retentate stream comprising 0.005 volume percent or less of carbon dioxide, and/or Wherein the second membrane stage has a surface area of the second gas separation membrane configured to provide a second retentate stream having a volumetric concentration of carbon dioxide reduced by 1/2000 or less relative to the volumetric concentration of carbon dioxide in the first retentate stream.
15. 15. The system of claim 12, wherein the heat exchanger cools the second retentate stream to form an at least partially liquefied second retentate stream, and wherein the system further comprises a flash tank in fluid flow communication with the heat exchanger for receiving, flashing, and separating the at least partially liquefied second retentate stream to form a flash gas and the liquefied methane product stream.
16. 16. The system of claim 15, wherein the flash tank is in fluid flow communication with the second membrane stage configured to receive a flash gas stream as a purge gas, whereby the flash gas stream is introduced as a purge gas into the shell space of the last gas separation module in the series.
17. 17. The system of claim 12, wherein the system is further configured such that the second permeate stream is recycled into the feed stream.
18. 18. The system of claim 12, wherein the system further comprises: A third membrane stage comprising a third gas separation membrane that is more permeable to carbon dioxide than methane, the third membrane stage configured to receive the first permeate stream and separate the stream into a third permeate stream that is further enriched in carbon dioxide and a third retentate stream.
19. 19. The system of claim 18, wherein the system is further configured such that the third retentate stream is recycled into the feed stream.
20. 20. The system of claim 12, wherein the system further comprises a compressor for compressing the feed stream prior to introducing the feed stream into the first membrane stage.

Description

Biological LNG production using membranes Cross Reference to Related Applications The present application claims priority from U.S. non-provisional application Ser. No. 18/499,285 filed on 1/11/2023, which is incorporated herein by reference. Background The present invention relates to a method and system for producing a liquefied methane product from a feed containing methane and carbon dioxide, wherein carbon dioxide is removed from the feed using a plurality of membrane stages including a gas separation membrane that is more permeable to carbon dioxide than to methane and forming a methane-enriched retentate stream that is then cooled and liquefied to provide the liquefied methane product. In particular, the present methods and systems may be used to produce liquefied biomethane (also referred to as "LBM" or "biological LNG") from a biogas feed. The use of membranes for biogas purification is well known in the art. There are a variety of system configurations to achieve a range of target product compositions (i.e., methane purity) and product recovery rates. Two and three membrane stage systems are most common (see e.g. US 8,999,038 and US 11,285,434). Most of these systems are intended to produce methane products comprising 2% or less by volume of carbon dioxide, wherein the methane recovery from a water saturated feed stream comprising 40% to 60% by volume of methane and 60% to 40% by volume of carbon dioxide is 95% or greater. Liquefied Natural Gas (LNG) production via cryogenic processes is also well known in the art. However, most LNG production uses pipeline quality gas or wellhead natural gas as feedstock. Pretreatment such as amine-based acid gas removal (i.e., removal of carbon dioxide using Selexol TM solvent and process) and drying using molecular sieve absorbers is required to remove the water and carbon dioxide content prior to introducing them into the liquefier cold box. Carbon dioxide and water must be removed to prevent ice or hydrates from forming in the cold box. US 10,254,041 discloses a system and method for processing hydrocarbon-containing fluids, such as, for example, biogas. The gaseous feed stream having a methane concentration of at least 50% by volume is purified using a membrane to provide a purified stream having a methane content of at least 85% by volume and a carbon dioxide content of between 0.1% and 2% by volume. The purified stream is then liquefied and flashed into a vessel from which is recovered (1) a liquid product stream having a methane concentration of at least 85% by volume, (2) a slurry stream of solid carbon dioxide and water ice, and (3) a flash gas stream. The flash gas may then be used as a sweep gas at the permeate side of one or more of the membranes. Disclosure of Invention Disclosed herein are methods and systems for producing liquefied methane products from methane and carbon dioxide containing feeds, such as, in particular, for producing liquefied biomethane (also referred to as "LBM" or "biological LNG") from biogas feeds. The disclosed methods and systems use multiple membrane stages including a gas separation membrane that is more permeable to carbon dioxide than methane to remove carbon dioxide from a feed and produce a methane-rich retentate stream that is then liquefied to provide a liquefied methane product. The membrane stage from which the methane-enriched retentate stream is discharged comprises a plurality of gas separation modules of hollow fiber type arranged in series, whereby the retentate gas discharged from the fibers of a gas separation module is mixed before being conveyed to the fibers of the next module in the series, and whereby the permeate gas discharged from the housing of a gas separation module is introduced as sweep gas into the housing space of the preceding gas separation module in the series. The disclosed methods and systems allow for the production of methane-rich retentate streams containing very low levels of carbon dioxide (e.g., 50 ppm (0.005 vol%) or less of CO 2) from a starting feed containing significant amounts of carbon dioxide using only membrane-based gas separation techniques. This in turn means that the disclosed methods and systems can produce liquefied biomethane (or other such) product from raw biogas (or other such) feed using only membrane units and liquefaction units without requiring any additional units or additional equipment for CO 2 removal, such as an amine scrubbing unit or TSA (temperature swing adsorption) unit for CO 2 removal upstream of the liquefaction unit or dedicated equipment for removal of frozen (frozen) CO 2 solids from within or downstream of the liquefaction unit. The disclosed methods and systems are also capable of producing a methane-enriched retentate stream containing very low levels of CO 2 using only membrane-based gas separation techniques via the specific arrangement of the membrane-stage gas separation module from which the methane-enriched retentate stream is discha