METHOD AND SYSTEM FOR PURIFICATION OF NATURAL GAS USING MEMBRANES

Abstract

Natural gas may be purified by removing C.sub.3+ hydrocarbons and CO.sub.2 in respective first and second gas separation membrane stages to yield conditioned gas lower in C.sub.3+ hydrocarbons and CO.sub.2 in comparison to the un-conditioned natural gas.

Claims

1. A method for purification of natural gas including methane, CO2, and C.sub.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 having a selective layer that is selective for C.sub.3+ hydrocarbons over methane; withdrawing a first permeate stream from the membrane(s) of the first stage that is enriched in C.sub.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.sub.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 having a selective layer that is selective for CO.sub.2 over methane; withdrawing a second permeate stream from the membrane(s) of the second stage that is enriched in CO.sub.2 in comparison to the feed gas; and withdrawing a first retentate stream from the membrane(s) of the first stage that is deficient in CO.sub.2 in comparison to the feed gas.

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 the one or membranes of the first gas separation membrane stage have a separation layer made of a copolymer or block polymer of tetramethylene oxide, and/or propylene oxide, or ethylene oxide.

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

10. The method of claim 8, wherein 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).

11. The method of claim 8, wherein 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).

12. The method of claim 8, 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.sup.2.Math.sec.Math.Pa).

13. The method of claim 8, wherein the one or more membranes of the first gas separation membrane stage have a methane permeance of less than 34 GPU.

14. The method of claim 8, wherein the one or more membranes of the first gas separation membrane stage have a methane permeance of less than 20 GPU.

15. The method of 8, wherein 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: ##STR00013## where PA is an aliphatic polyamide having 6 or 12 carbon atoms and PE is either poly(ethylene oxide) poly(tetramethylene oxide).

16. The method of claim 8, wherein one or membranes of the first gas separation membrane stage have a separation layer made of repeating units of the following monomers: ##STR00014##

17. The method of claim 8, 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.

18. The method of claim 8, 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.

19. The method of claim 18, wherein each of the support layers is made of a polyimide, polysulfone, or polyether ether ketone.

20. The method of claim 19, wherein each of the support layers is porous and is made of polyether ether ketone.

21. 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.

22. A system for purification of natural gas including methane, CO2, and C.sub.3+ hydrocarbons, comprising: a source of natural gas; a first gas separation membrane stage comprising one or more membranes fluidly communicating with said source, each membrane of the first gas separation membrane stage having a selective layer that is selective for C.sub.3+ hydrocarbons over methane; and a second gas separation membrane stage comprising one or more membranes 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.sub.2 over methane.

23. The system of claim 22, 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.

24. The system of claim 23, wherein said water removal apparatus is a molecular sieve adapted and configured to remove water from fluids.

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

26. The system of claim 22, wherein each of the one or membranes of the first gas separation membrane stage has a separation layer made of a copolymer or block polymer of tetramethylene oxide, and/or propylene oxide, or ethylene oxide.

27. The system of claim 26, 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 50 psi (3.45 bar).

28. The system of claim 26, 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).

29. The system of claim 26, 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).

30. The system of claim 26, wherein each of the one or membranes of the first gas separation membrane stage exhibits a methane permeance of less than 68 gas permeation units (22.4 mol/m.sup.2.Math.sec.Math.Pa).

31. The system of claim 26, wherein each of the one or membranes of the first gas separation membrane stage exhibits a methane permeance of less than 34 GPU.

32. The system of claim 26, wherein each of the one or membranes of the first gas separation membrane stage exhibits a methane permeance of less than 20 GPU.

33. The system of claim 26, wherein 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: ##STR00015## where PA is an aliphatic polyamide having 6 or 12 carbon atoms and PE is either poly(ethylene oxide) poly(tetramethylene oxide).

34. The system of claim 26, wherein one or membranes of the first gas separation membrane stage have a separation layer made of repeating units of the following monomers: ##STR00016##

35. The system of claim 26, 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.

36. The system of claim 26, 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.

37. The system of claim 36, wherein each of the support layers is made of a polyimide, polysulfone, or polyether ether ketone.

38. The system of claim 37, wherein each of the support layers is porous and is made of polyether ether ketone.

39. The system of claim 22, wherein each of the membranes of the second gas separation membrane stage is made of cellulose acetate, a polysulfone, or a polyimide.

Description

DESCRIPTION OF PREFERRED EMBODIMENTS

[0033] Natural gas may be conditioned with gas separation membranes so as to meet desired levels of C3+ hydrocarbons, CO.sub.2, and optionally H.sub.2S. The unconditioned gas may optionally be pre-treated with a molecular sieve (or equivalent dehydration technique) upstream of the gas separation membranes in order to dry the unconditioned gas prior to membrane separation. The conditioning process includes feeding the feed gas (i.e., the unconditioned natural gas which has optionally been dehydrated with a molecular sieve or equivalent dehydration technique) to a first gas separation membrane stage.

[0034] The membranes of the first gas separation membrane stage include a selective layer that is selective for C.sub.3+ hydrocarbons over methane. A first stage permeate stream is withdrawn from a permeate side of the first stage membrane and a first stage retentate stream is withdrawn from the feed gas side of the first stage membrane. By selective for C.sub.3+ hydrocarbons over methane, we mean that, as a whole, the C.sub.3+ hydrocarbons become enriched in the permeate stream in comparison to the feed gas and the C.sub.3+ hydrocarbons dewpoint of the retentate is lowered. Those skilled in the art of gas separation membrane technology will recognize that the C.sub.3+ hydrocarbons dewpoint is the temperature at which cooling of the retentate will cause condensation of C.sub.3+ hydrocarbons.

[0035] The first retentate stream is fed to a second gas separation membrane stage. The membranes of the second gas separation membrane stage include a selective layer that is selective for CO2 over methane. A second stage permeate stream is withdrawn from a permeate side of the second stage membrane and a second stage retentate stream is withdrawn from the feed gas side of the second stage membrane.

[0036] If flaring of the first and/or second stage permeate streams is prohibited due to environmental regulations or if it is economical or otherwise desirable to not flare such streams, it may be re-injected deep underground (or in the case of subsea natural gas extraction, deep under the seabed). In the event that the first and/or second stage permeate stream contains too high of a moisture content to allow re-injection as is, such a stream may first be dehydrated by any suitable technique for gas dehydration to reach a moisture content of no more than 50 ppm (vol/vol) and as low as 1 ppm (vol/vol).

[0037] If flaring is otherwise allowable and desired instead of re-injection, the first and/or second stage permeate stream may be combusted as a flare gas with or without additional separate flare gases associated with other gases collected in the natural gas extraction and conditioning processes.

[0038] The separation layer of the first gas separation membrane stage may be made of a copolymer or block polymer of tetramethylene oxide, and/or propylene oxide, or ethylene oxide. These types of polymers exhibit modest productivity (i.e., permeance) for methane and preferential permeation of C.sub.3+ hydrocarbons. Due to the modest methane productivity of these polymers in comparison with silicone based polymers, membranes with low methane productivity for methane can be conveniently achieved. Through selection of a first gas separation stage membrane separation layer with modest methane productivity and preferential permeation of C.sub.3+ hydrocarbons, only a relatively low pressure drop across the first membrane stage (i.e., the difference in pressure between the feed gas and the retentate gas) may be realized. As a result, there is no need for recompression of the first retentate before it is fed to the second stage. Typically, the pressure drop between the feed gas and the retentate gas is less than 50 psi (3.45 bar). The pressure drop may be less than 30 psi (2.07 bar) or even less than 20 psi (1.38 bar). Typically, the membrane productivity for methane should be below 68 GPU (22.4 mol/m.sup.2.Math.sec.Math.Pa). Often, it is below 34 GPU or even below 20 GPU.

[0039] Copolymers or block polymers of tetramethylene oxide, and/or propylene oxide, or ethylene oxide may be conveniently synthesized, such as the polyester ether disclosed in U.S. Pat. No. 6,860,920, the polyester ethers of which are incorporated by reference.

##STR00003##

where PE may be one or more of the following structures:

##STR00004##

[0040] Other copolymers or block polymers of tetramethylene oxide, and/or propylene oxide, or ethylene oxide may be conveniently synthesized, such as polyimide ether disclosed in U.S. Pat. No. 5,776,990, the polyimide ethers of which are incorporated by reference.

[0041] The copolymers can be further obtained by copolymerization of acrylated monomers containing oligomeric propylene oxide, ethyelene oxide, or tetramethyelene oxide. Commercially available copolymers include poly(ether-b-amide) multiblock copolymers available from Arkema under the trade name of PEBAX, and poly(butylene terephthalate) ethylene oxide copolymer available under the trade name of Polyactive.

[0042] Typically, the PEBAX polymers from Arkema include PEBAX 7233, PEBAX 7033, PEBAX 6333, PEBAX 2533, PEBAX 3533, PEBAX 1205, PEBAX 3000, PEBAX 1657, or PEBAX 1074. PEBAX 1657 exhibits a methane permeability of 5.12 Barrer. H. Rabiee, et al., J. Membrane Sci. vol. 476, pp. 286-302 (2015). In contrast, PDMS exhibits a methane permeability of 800 Barrer. Stern, et al., J. Appl. Polym. Sci., Vol. 38, 2131(1989). The PEBAX polymers have the following general chemical structure:

##STR00005##

Where PA is an aliphatic polyamide hard block (nylon 6 [PA6] or nylon 12 [PA12], and PE denotes a polyether soft block, either poly(ethylene oxide) [PEO] or poly(tetramethylene oxide) [PTMEO].

[0043] Commercial available PolyActive multiblock copolymers have the following general chemical structure:

##STR00006##

[0044] While the membranes of the first gas separation membrane may have any configuration known in the field of gas separation, typically they are formed as a flat film or as a plurality of hollow fibers. In one embodiment, the separation layer is supported by a support layer where the separation layer performs the desired separation while the support layer provides mechanical strength. In the context of hollow fibers, the separation layer is configured as a sheath surrounding a core made of the support layer. Regardless of the configuration of the membrane, the support layer may be any porous substrate known in the field of gas separation membranes and includes but is not limited to, polyimides, polysulfones, and polyether ether ketones. Typical hollow fiber membrane supports are PEEK porous substrate fibers commercially available from Air Liquide.

[0045] Typically, the first gas separation membrane stage includes membranes commercially available from Medal under the trade name PEEK-SEP. The separation layer of the second gas separation membrane stage may be made of any polymer or copolymer known in the field of gas separation membranes that is selective for CO.sub.2 over methane. Typically, the separation layer of the second gas separation membrane stage is made of cellulose acetate, a polysulfone, or a polyimide. Typically, the polyimide essentially consists of repeating units of dianhydride-derived units of formula (I) and diamine-derived units.

##STR00007##

Each R is a molecular segment independently selected from the group consisting of formula (1), formula (2), formula (3), and formula (4):

##STR00008##

Each Z is a molecular segment independently selected from the group consisting of formula (5), formula (6), formula (7), formula (8), and formula (9).

##STR00009##

Each diamine-derived unit is a diamine-derived moiety independently selected from the group consisting of formula (A), formula (B), formula (C), formula (D), formula (E), formula (F), formula (G), and formula (H):

##STR00010##

Each X, X.sub.1, X.sub.2, X.sub.3, X.sub.4, X.sub.5, X.sub.6, X.sub.7, and X.sub.8 is independently selected from the group consisting of hydrogen, an aromatic group, and a straight or branched C.sub.1 to C.sub.6 alkyl group. Each R.sub.a is a straight or branched C.sub.1 to C.sub.6 alkyl group having either a terminal hydroxyl group, a terminal carboxylic acid group, or a terminal carbon to carbon double bond. Each Z is a molecular segment selected from the group consisting of formula (a), formula (b), formula (c), and formula (d):

##STR00011##

Each Z is a moiety selected from the group consisting of formula (U) and formula (V):

##STR00012##

Each X.sub.9 is selected from the group consisting of hydrogen, a straight or branched alkyl group having 1 to 6 carbon atoms, and a straight or branched pefluoroalkyl group having 1 to 6 carbon atoms.

[0046] While the membranes of the first gas separation membrane may have any configuration known in the field of gas separation, typically they are formed as a flat film or as a plurality of hollow fibers. In one embodiment, the separation layer is supported by a support layer where the separation layer performs the desired separation while the support layer provides mechanical strength. In the context of hollow fibers, the separation layer is configured as a sheath surrounding a core made of the support layer. Regardless of the configuration of the membrane, the support layer may be any porous substrate known in the field of gas separation membranes. Suitable membranes for the second gas separation membrane stage are commercially available from Medal, a unit of Air Liquide Advanced Technologies, US.

Prophetic Examples

[0047] Example: A computer simulation was performed in order to demonstrate the process of the invention. In the simulation, a feed gas with the following gas composition was fed into a composite membrane including a PEBAX separation layer and a PEEK support layer with methane permeance of 15 GPU at 1000 psia and 30 C. The membrane cartridge exhibits a pressure drop of only 37 psi.

TABLE-US-00001 FEED RAFF PERM F, MMSCFD(60 F.) 1.257 1 0.2567 PRESS, psia 1000 963.88 26.3 CONCENTRATIONS, mol % WATER 0.1991 0.0043 0.9582 CARBON_DIOXIDE 44.9649 37.0415 75.8347 NITROGEN 0.4978 0.6132 0.0486 ETHANE 5.5858 5.9936 3.9967 PROPANE 3.6243 3.7977 2.9486 N-BUTANE 1.613 1.4971 2.0646 N-PENTANE 0.4978 0.3258 1.1681 N-HEXANE 0.2091 0.1007 0.6313 METHANE 42.8082 50.6262 12.3492

[0048] Comparative Example 2: A computer simulation was also attempted for the purpose of demonstrating a process that is not of the invention. A silicone based membrane with methane permeance of 120 GPU is used. The same feed condition as in the Example was used for the calculation. The pressure drop is so significant that the calculation did not converge.

[0049] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0050] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0051] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

[0052] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0053] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0054] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0055] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.