Method and system for purification of natural gas using membranes
Natural gas may be purified by removing C 3+ hydrocarbons and CO 2 in respective one or more separation units to yield conditioned gas lower in C 3+ hydrocarbons and CO 2 in comparison to the un-conditioned natural gas. Notably, the feed gas need not be subjected to joule-thomson expansion and molecular sieve dehydration performed by conventional processes. Rather, any water-rich reject stream from the separation unit(s) is dried downstream with a smaller compressor and smaller molecular sieve or gas separation membrane dehydration unit before it may be re-injected deep underground or deep under the sea bed.
1. A method for purification of natural gas including methane, CO 2 , water, and C 3+ hydrocarbons, comprising the steps of:
feeding a feed gas consisting of the natural gas to a first separation unit;
withdrawing a a first retentate stream from the first separation unit that is enriched in methane in comparison to the feed gas and deficient in C 3+ hydrocarbons, CO 2 , and water in comparison to the feed gas;
withdrawing a first permeate stream from the first separation unit that is enriched in C 3+ hydrocarbons, water, and CO 2 in comparison to the feed gas and deficient in methane in comparison to the feed gas;
feeding the first retentate stream to a second separation unit;
withdrawing a dry conditioned natural gas from the second separation unit that is enriched in methane in comparison to the first retentate stream and deficient in C 3+ hydrocarbons, CO 2 and water in comparison to the first retentate stream;
withdrawing a second permeate stream from the second separation unit that is enriched in C 3+ hydrocarbons, water, and CO 2 in comparison to the first retentate stream and deficient in methane in comparison to the first retentate stream;
compressing the first permeate stream so as to cause condensation of least some of the water contained therein to produce a biphasic stream having liquid and gaseous phases;
removing the liquid phase from the biphasic stream in a phase separator; and
feeding the gaseous phase from the phase separator to a dehydration apparatus so as to remove at least some of the water contained therein and produce a stream of the dried gaseous phase.
2. The method of claim 1 , wherein the feed gas is extracted from a subsea or subterranean oil or gas field has not been dehydrated after extraction therefrom.
3. The method of claim 1 , wherein the dry conditioned natural gas meets pipeline specifications for natural gas or fuel specifications for natural gas-powered heavy equipment.
4. The method of claim 1 , wherein the dry conditioned natural gas has a carbon dioxide concentration below 3% (vol/vol) and a C 3+ hydrocarbon dewpoint of no more than −20° C.
5. The method of claim 1 , wherein the dried gaseous phase has a water content of no more than 2 ppm.
6. The method of claim 1 , wherein each of the first and second separation units comprises an adsorbent bed, at least one of the adsorbent beds exhibits adsorptive affinity for C 3+ hydrocarbons over methane, at least one of the adsorbent beds exhibits adsorptive affinity for water over methane, and at least one of the adsorbent beds exhibits adsorptive affinity for CO 2 over methane.
7. The method of claim 1 , wherein the first and second separation units comprise first and second gas separation membrane units, respectively, the first gas separation membrane unit comprises one or more membranes in parallel or in series each of which has a selective layer that is selective for C 3+ hydrocarbons over methane and water over methane, and the second gas separation unit comprises one or more membranes in parallel or in series each of which has a selective layer that is selective for CO 2 over methane.
8. The method of claim 7 , wherein the selective layer of each of the gas separation membranes of the first gas separation membrane unit comprises a polymer or copolymer including repeating units of a monomer selected from the group consisting of tetramethylene oxide, propylene oxide, and ethylene oxide.
9. The method of claim 8 , wherein the polymer or copolymer comprises repeating units according to formula (I):
wherein:
PA is an aliphatic polyamide according to formula (II) or formula (III):
and
PE is selected from the group consisting of poly(ethylene oxide) and poly(tetramethylene oxide).
10. The method of claim 8 , wherein the polymer or copolymer comprises copolymers obtained by copolymerization of acrylated monomers containing oligomeric propylene oxide, ethylene oxide, or tetramethyelene oxide.
11. The method of claim 8 , wherein the polymer or copolymer comprises a copolymer copolymerized from the monomers according to formulae (IV) and (V):
where x+y=1.
12. The method of claim 8 , wherein each of the gas separation membranes of the first gas separation membrane unit has a membrane productivity for methane of below 20 GPU.
13. The method of claim 7 , wherein a pressure drop between the feed gas and the water-deficient and C 3+ hydrocarbons-deficient stream of the first gas separation membrane unit is less than 50 psi (3.45 bar).
14. The method of claim 7 , wherein the selective layer is supported by a support layer comprising poly(ether ether ketone).
15. The method of claim 1 , wherein the dehydration apparatus comprises a molecular sieve.
16. The method of claim 1 , wherein the dehydration apparatus comprises a gas separation unit comprising one or more gas separation membranes that are selective for water over CO 2 and selective for water over C 3+ hydrocarbons.
17. The method of claim 7 , wherein the selective layer of each of the gas separation membranes of the second gas separation membrane unit comprises a polymer or copolymer selected from the group consisting of polyimides, cellulose acetate and polysulfone.