Multi-stage membrane for N2 rejection

Patent metadata

Field	Value
Publication number	US-10780392-B2
Application number	US-201815959816-A
Country	US
Kind code	B2
Filing date	Apr 23, 2018
Priority date	Apr 23, 2018
Publication date	Sep 22, 2020
Grant date	Sep 22, 2020

Abstract

Nitrogen is removed from biogas using a three-stage separation system based on gas separation membranes. The first stage separates a biomethane feed stream into a first permeate gas stream and a first retentate gas stream. The second stage separates the first permeate stream into a biomethane product gas and a first low quality biomethane gas stream. The third stage separates the first retentate into a second low quality biomethane gas stream and a waste gas. A biogas feed stream is pretreated to remove amounts of water, VOCs, and CO 2 to yield a methane-enriched biogas stream. The methane-enriched biogas stream is compressed together with the first and second low quality biomethane gas streams to form the biomethane feed stream.

First claim

The invention claimed is:

1. A method for purifying biogas, comprising the steps of:
pretreating a biogas feed stream so as to remove amounts of water, VOCs, and CO2 from the biogas feed stream to produce a methane-enriched biogas stream that comprises methane, CO 2 , O 2 , and at least 5 vol % N 2 ;
separating a biomethane feed stream at a first separation stage into a first permeate gas stream comprising at least 90 vol % CH 4 and less than 8 vol % N 2 and a first retentate gas stream comprising at least 10 vol % N 2 , the first separation stage comprising at least one polymeric gas separation membrane selective for methane over N 2 ;
separating said first permeate gas stream at a second separation stage comprising at least one polymeric gas separation membrane into a second permeate gas stream comprising at least 94 vol % CH 4 and less than 5 vol % N 2 and a second retentate gas stream comprising at least 7 vol % N 2 , the second separation stage comprising at least one polymeric gas separation membrane selective for methane over N 2 ;
separating said first retentate gas stream at a third separation stage comprising at least one polymeric gas separation membrane into a third permeate gas stream comprising at least 80 vol % CH 4 and less than 20 vol % N 2 and a third retentate gas stream comprising at least 40 vol % N 2 , the third separation stage comprising at least one polymeric gas separation membrane selective for methane over N 2 ;
feeding said methane-enriched biogas stream, second retentate gas stream and third permeate gas stream to a compression station comprising one or more compressors where they are combined and compressed to form the biomethane feed stream; and
withdrawing said second permeate gas stream as biomethane product gas.

2. The method of claim 1 , further comprising the step of compressing said biomethane product gas in a secondary compressor.

3. The method of claim 1 , wherein said methane-enriched biogas stream comprises at least 85 vol % methane and impurities comprising at least 5 vol % and up to 15 vol % N 2 , a non-zero amount of O 2 up to 1 vol % O 2 ; a non-zero amount of CO 2 but less than 2 vol %; no more than 0.05 wt. % water, and less than 100 ppm each of H 2 S and VOCs.

4. The method of claim 1 , wherein at least one of said at least one membrane of said first, second and third separation stages is comprised of a porous polymeric substrate having at least one separation layer, wherein said substrate is selected from the group consisting of polyimides, poly sulfones, polyether ether ketones, and mixtures thereof.

5. The method of claim 4 , wherein said substrate is polyether ether ketones.

6. The method of claim 4 , wherein said at least one separation layer is made of a copolymer or block polymer of the formula:

where PA is an aliphatic polyamide having 6 or 12 carbon atoms and PE is either poly(ethylene oxide) or poly(tetramethylene oxide).

7. The method of claim 4 , wherein said separation layer is a polymer made of repeating units of the following monomers:

8. The method of claim 4 , wherein said separation layer is a copolymer or block polymer made up of monomeric units of dimethylsiloxane, methylhexylsiloxane, methylphenylsiloxane, methyloctylsiloxane, or combinations thereof.

9. The method of claim 4 , wherein said separation layer is a polymer made up of monomeric units of tetramethylene oxide, propylene oxide, ethylene oxide, or combinations thereof.

10. The method of claim 1 , wherein each of said at least one membrane of said first, second and third separation stages has a CH 4 /N 2 selectivity of 1.5 to 5.5.

11. The method of claim 1 , wherein a pressure differential between said biomethane feed stream and said first permeate gas stream is at least 100 psig.

12. The method of claim 11 , wherein said pressure differential is at least 150 psig.

13. The method of claim 1 , wherein said first permeate gas stream comprises at least 92 vol % methane, less than 1 vol % carbon dioxide, less than 7 vol % nitrogen, and less than 0.05 wt. % water.

14. The method of claim 1 , further comprising the step of liquefying the biomethane product gas.

15. The method of claim 1 , wherein said first retentate gas comprises at least 80 vol % methane, less than 20 vol % nitrogen, less than 0.5 vol % CO 2 , and less than 0.05 wt. % water.

16. The method of claim 1 , further comprising the step of flaring said third retentate gas stream.

17. The method of claim 1 , further comprising the step of oxidizing the third retentate gas stream in a thermal oxidizer.

18. The method of claim 1 , further comprising the step of using the third retentate stream as a fuel in an internal combustion engine.

19. The method of claim 1 , wherein said biomethane product gas includes N 2 at a concentration of no more than 3 vol %, CO2 at a concentration of less than 2 vol %, and H 2 O at a concentration of less than 0.01 wt. %.

20. A method for purifying biogas, comprising the steps of:
pretreating a biogas feed stream so as to remove amounts of water, VOCs, and CO2 from the biogas feed stream to produce a methane-enriched biogas stream that comprises methane, CO 2 , O 2 , and at least 5 vol % N 2 ;
separating a biomethane feed stream at a first separation stage into a first permeate gas stream comprising at least 90 vol % CH 4 and less than 8 vol % N 2 and a first retentate gas stream comprising at least 10 vol % N 2 , the first separation stage comprising at least one polymeric gas separation membrane selective for methane over N 2 ;
separating said first permeate gas stream at a second separation stage comprising at least one polymeric gas separation membrane into a second permeate gas stream comprising at least 94 vol % CH 4 and less than 5 vol % N 2 and a second retentate gas stream comprising at least 7 vol % N 2 , the second separation stage comprising at least one polymeric gas separation membrane selective for methane over N 2 ;
separating said first retentate gas stream at a third separation stage comprising at least one polymeric gas separation membrane into a third permeate gas stream comprising at least 80 vol % CH 4 and less than 20 vol % N 2 and a third retentate gas stream comprising at least 40 vol % N 2 , the third separation stage comprising at least one polymeric gas separation membrane selective for methane over N 2 ;
compressing the second stage permeate gas stream;
separating said compressed second stage permeate gas stream at a fourth separation stage comprising at least one glassy gas separation membrane, each having a N 2 /methane selectivity of at least 1.5, into a fourth permeate gas stream and fourth retentate gas stream;
feeding said methane-enriched biogas stream, second retentate gas stream, third permeate gas stream, and fourth permeate gas stream to a compression station comprising one or more compressors where they are combined and compressed to form the biomethane feed stream; and
withdrawing said fourth retentate gas stream as biomethane product gas.

21. A system for purifying biogas, comprising:
a pretreatment system comprising an inlet, an outlet, and one or more gas purification apparatuses adapted and configured to remove amount of water, VOCs, and CO 2 from a biogas feed stream;
a compression station, comprising an inl

Assignees

• Air Liquide Advanced Tech Us Llc

Inventors

• Bikson Benjamin
• Ding Yong
• Mitariten Michael J

Classifications

• B01D71/64

  Polyimides; Polyamide-imides; Polyester-imides; Polyamide acids or similar polyimide precursors · CPC title

• B01D71/60

  Polyamines · CPC title

• B01D71/701

  Polydimethylsiloxane · CPC title

• B01D71/5211

  Polyethylene glycol or polyethyleneoxide · CPC title

• B01D2257/504

  Carbon dioxide · CPC title