Method and system for purification of natural gas using membranes

1. A method for purification of natural gas including methane, CO 2 , and C 3+ hydrocarbons, comprising the steps of:

feeding a feed gas consisting of the natural gas to a first gas separation membrane stage comprising one or more membranes in series or parallel having a selective layer that is selective for C 3+ hydrocarbons over methane;

withdrawing a first permeate stream from the membrane(s) of the first stage that is enriched in C 3+ hydrocarbons in comparison to the feed gas;

withdrawing a first retentate stream from the membrane(s) of the first stage that is deficient in C 3+ hydrocarbons in comparison to the feed gas;

feeding the first retentate stream to a second gas separation membrane stage comprising one or more membranes in series or parallel having a selective layer that is selective for CO 2 over methane;

withdrawing a second permeate stream from the membrane(s) of the second stage that is enriched in CO 2 in comparison to the feed gas; and

withdrawing a second retentate stream from the membrane(s) of the second stage that is deficient in CO 2 in comparison to the feed gas, wherein:

the one or more membranes of the first gas separation membrane stage have a methane permeance of less than 68 gas permeation units (22.4 mol/m 2 ·sec·Pa); and

the one or membranes of the first gas separation membrane stage have a separation layer made of: 1) a copolymer or block polymer of tetramethylene oxide, 2) a copolymer or block polymer of tetramethylene oxide and propylene oxide, 3) a copolymer or block polymer of propylene oxide, or 4) a copolymer or block polymer of ethylene oxide, terephthalate, and tetramethylene terephthalate.

2. The method of claim 1 , further comprising removing water from the feed gas prior to feeding the feed gas to the first gas separation membrane stage.

3. The method of claim 2 , wherein said step of removing water comprises feeding the feed gas to a molecular sieve adapted and configured to remove water from fluids.

4. The method of claim 2 , wherein said step of removing water comprises feeding the feed gas to a dehydration gas separation membrane.

5. The method of claim 1 , further comprising the step of combusting the first and/or the second permeate streams as a flare gas.

6. The method of claim 1 , wherein the feed gas is obtained from natural gas extracted from a subterranean or subsea geological formation and said step further comprises injecting the first and/or second stage permeate streams into the geological formation.

7. The method of claim 6 , further comprising dehydrating the first and/or second permeate streams prior to injection into the geological formation such that a water content in the first and/or second permeate stream injected into the geological formation is no more than 50 ppm (vol/vol).

8. The method of claim 1 , wherein a pressure drop between a pressure of the feed gas and a pressure of the first retentate stream is less than 50 psi (3.45 bar).

9. The method of claim 1 , wherein a pressure drop between a pressure of the feed gas and a pressure of the first retentate stream is less than 30 psi (2.07 bar).

10. The method of claim 1 , wherein a pressure drop between a pressure of the feed gas and a pressure of the first retentate stream is less than less than 20 psi (1.38 bar).

11. The method of claim 1 , wherein the one or more membranes of the first gas separation membrane stage have a methane permeance of less than 34 GPU.

12. The method of claim 1 , wherein the one or more membranes of the first gas separation membrane stage have a methane permeance of less than 20 GPU.

13. A method for purification of natural gas including methane, CO 2 , and C 3+ hydrocarbons, comprising the steps of:

feeding a feed gas consisting of the natural gas to a first gas separation membrane stage comprising one or more membranes in series or parallel having a selective layer that is selective for C 3+ hydrocarbons over methane;

withdrawing a first permeate stream from the membrane(s) of the first stage that is enriched in C 3+ hydrocarbons in comparison to the feed gas;

withdrawing a first retentate stream from the membrane(s) of the first stage that is deficient in C 3+ hydrocarbons in comparison to the feed gas;

feeding the first retentate stream to a second gas separation membrane stage comprising one or more membranes in series or parallel having a selective layer that is selective for CO 2 over methane;

withdrawing a second permeate stream from the membrane(s) of the second stage that is enriched in CO 2 in comparison to the feed gas; and

withdrawing a second retentate stream from the membrane(s) of the second stage that is deficient in CO 2 in comparison to the feed gas, wherein:

the one or more membranes of the first gas separation membrane stage have a methane permeance of less than 68 gas permeation units (22.4 mol/m 2 ·sec·Pa); and

the one or membranes of the first gas separation membrane stage have a separation layer 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) poly(tetramethylene oxide).

14. The method of claim 1 , wherein one or membranes of the first gas separation membrane stage have a separation layer made of repeating units of the following monomers:

15. The method of claim 1 , wherein the one or more membranes of the first gas separation membrane stage are formed as flat films or as a plurality of hollow fibers.

16. The method of claim 1 , wherein each of the one or more membranes of the first gas separation membrane stage has a separation layer that is supported by a support layer.

17. The method of claim 16 , wherein each of the support layers is made of a polyimide, polysulfone, or polyether ether ketone.

18. The method of claim 17 , wherein each of the support layers is porous and is made of polyether ether ketone.

19. The method of claim 1 , wherein each of the membranes of the second gas separation membrane stage is made of cellulose acetate, a polysulfone, or a polyimide.

20. A system for purification of natural gas including methane, CO2, and C 3+ hydrocarbons, comprising:

a source of natural gas;

a first gas separation membrane stage comprising one or more membranes fluidly in series or parallel communicating with said source, each membrane of the first gas separation membrane stage having a selective layer that is selective for C 3+ hydrocarbons over methane; and

a second gas separation membrane stage comprising one or more membranes in series or parallel fluidly communicating with a retentate outlet(s) of the membranes of the first gas separation membrane stage so as to receive retentate from the first gas separation membrane stage as a feed gas in the second gas separation membrane stage, each membrane of the second gas separation membrane stage having a selective layer that is selective for CO 2 over methane, wherein:

the one or more membranes of the first gas separation membrane stage have a methane permeance of less than 68 gas permeation units (22.4 mol/m 2 ·sec·Pa); and

the one or membranes of the first gas separation membrane stage have a separation layer made of: 1) a copolymer or block polymer of tetramethylene oxide, 2) a copolymer or block polymer of tetramethylene oxide and propylene oxide, 3) a copolymer or block polymer of propylene oxide, or 4) a copolymer or block polymer of ethylene oxide, terephthalate, and tetramethylene terephthalate.

21. The system of claim 20 , further comprising a water removal apparatus adapted and configured to remove water from the feed gas prior to feeding the feed gas to the first gas separation membrane stage.

22. The system of claim 21 , wherein said water removal apparatus is a molecular sieve adapted and configured to remove water from fluids.

23. The system of claim 21 , wherein said water removal apparatus is a dehydration gas separation membrane.

24. The system of claim 20 , wherein each of the one or membranes of the first gas separation membrane stage exhibits a pressure drop between a pressure of the feed gas and a pressure of the retentate gas of less than 50 psi (3.45 bar).

25. The system of claim 20 , wherein each of the one or membranes of the first gas separation membrane stage exhibits a pressure drop between a pressure of the feed gas and a pressure of the retentate gas is less than 30 psi (2.07 bar).

26. The system of claim 20 , wherein each of the one or membranes of the first gas separation membrane stage exhibits a pressure drop between a pressure of the feed gas and a pressure of the retentate gas is less than less than 20 psi (1.38 bar).

27. The system of claim 20 , wherein each of the one or membranes of the first gas separation membrane stage exhibits a methane permeance of less than 34 GPU.

28. The system of claim 20 , wherein each of the one or membranes of the first gas separation membrane stage exhibits a methane permeance of less than 20 GPU.

29. A system for purification of natural gas including methane, CO2, and C 3+ hydrocarbons, comprising:

a source of natural gas;

a first gas separation membrane stage comprising one or more membranes fluidly in series or parallel communicating with said source, each membrane of the first gas separation membrane stage having a selective layer that is selective for C 3+ hydrocarbons over methane; and

a second gas separation membrane stage comprising one or more membranes in series or parallel fluidly communicating with a retentate outlet(s) of the membranes of the first gas separation membrane stage so as to receive retentate from the first gas separation membrane stage as a feed gas in the second gas separation membrane stage, each membrane of the second gas separation membrane stage having a selective layer that is selective for CO 2 over methane, wherein:

the one or more membranes of the first gas separation membrane stage have a methane permeance of less than 68 gas permeation units (22.4 mol/m 2 ·sec·Pa); and

the one or membranes of the first gas separation membrane stage have a separation layer 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) poly(tetramethylene oxide).

30. The system of claim 20 , wherein one or membranes of the first gas separation membrane stage have a separation layer made of repeating units of the following monomers:

31. The system of claim 20 , wherein the one or more membranes of the first gas separation membrane stage are formed as flat films or as a plurality of hollow fibers.

32. The system of claim 20 , wherein each of the one or more membranes of the first gas separation membrane stage has a separation layer that is supported by a support layer.

33. The system of claim 32 , wherein each of the support layers is made of a polyimide, polysulfone, or polyether ether ketone.

34. The system of claim 33 , wherein each of the support layers is porous and is made of polyether ether ketone.

35. The system of claim 20 , wherein each of the membranes of the second gas separation membrane stage is made of cellulose acetate, a polysulfone, or a polyimide.