Open loop liquefaction process with NGL recovery
Described herein are methods and systems for removing natural gas liquids from a natural gas feed stream and for liquefying the natural gas feed stream so as to produce a liquefied natural gas (LNG) stream and a natural gas liquids (NGL) stream.
1. A method for removing natural gas liquids from and liquefying a natural gas feed stream, the method comprising the steps of:
(a) expanding and/or cooling a natural gas feed stream and introducing said stream into a distillation column having one or more separation sections, the natural gas feed stream being introduced into the distillation column below at least one of said separation sections;
(b) withdrawing a natural gas liquids stream from the bottom of the distillation column;
(c) withdrawing a natural gas vapor stream from the top of the distillation column
(d) warming the natural gas vapor stream and a first expanded refrigerant stream in one or more heat exchanger sections, compressing the resulting warmed streams, and combining said streams to form a compressed refrigerant, wherein the natural gas vapor stream and first expanded refrigerant stream are may be combined prior to, during or after being warmed and compressed;
(e) cooling a first portion of the compressed refrigerant and a second portion of the compressed refrigerant, via indirect heat exchange with the natural gas vapor stream and the first expanded refrigerant stream that are being warmed in step (d), to form respectively a first cold refrigerant stream and a second cold refrigerant stream, the first and second portions of the compressed refrigerant being cooled against the natural gas vapor stream and the first expanded refrigerant stream and the first portion of the compressed refrigerant being then further cooled against the natural gas vapor stream and the first expanded refrigerant stream such that the first cold refrigerant stream is formed at a lower temperature than the second cold refrigerant stream;
(f) expanding the first cold refrigerant stream, expanding the second cold refrigerant stream, and combining and separating the resulting expanded streams into vapor and liquid phases to form a first liquefied natural gas stream from the liquid phase and the first expanded refrigerant stream from the vapor phase;
(g) forming a reflux stream and expanding and introducing the reflux stream into the top of the distillation column to provide reflux to the distillation column, wherein the reflux stream is formed from a portion of the first liquefied natural gas stream, a portion of the liquid phase separated in step (f), a portion of the first cold refrigerant stream withdrawn from said stream prior to said stream being separated in step (f), a further portion of the compressed refrigerant that has been cooled via indirect heat exchange with the natural gas vapor stream and the first expanded refrigerant stream that are being warmed in step (d), and/or a portion of a liquefied natural gas stream or liquefied natural gas product derived from the first liquefied natural gas stream.
2. The method of claim 1 , wherein in step (a) the distillation column into which the natural gas feed stream is introduced has two or more separation sections, the expanded natural gas feed stream being introduced into the distillation column below at least one of said separation sections and above at least another one of said separation sections.
3. The method of claim 2 , wherein the method further comprises the step of:
(h) providing boil-up to the distillation column by re-boiling a portion of the distillation column bottoms liquid.
4. The method of claim 1 , wherein in step (a) the natural gas feed stream is expanded before being introduced into the distillation column.
5. The method of claim 4 , wherein in step (a) the natural gas feed stream is cooled and then expanded before being introduced into the distillation column, wherein after being cooled the natural gas feed stream is separated into vapor and liquid phases with the vapor phase being expanded and introduced into the distillation column at a first location below at least one separation section of the column, and with the liquid phase being expanded and introduced into the distillation column at a second location below the first location, there being at least one separation section between the first and second locations.
6. The method of claim 1 , wherein in step (a) the natural gas feed stream is cooled before being introduced into the distillation column, at least a portion the natural gas feed stream being cooled via indirect heat exchange with the natural gas vapor stream and the first expanded refrigerant stream that are being warmed in step (d).
7. The method of claim 1 , wherein in step (g) the reflux stream is formed from a portion of the first liquefied natural gas stream and/or a portion of the liquid phase separated in step (f).
8. The method of claim 1 , wherein the first expanded refrigerant stream is formed at a lower temperature than the natural gas vapor stream, and wherein in step (e) the at least a first portion of the compressed refrigerant is cooled via indirect heat exchange with the natural gas vapor stream and the first expanded refrigerant stream and then further cooled via indirect heat exchange with the first expanded refrigerant stream to form the first cold refrigerant stream.
9. The method of claim 1 , wherein the method further comprises the step of:
(i) expanding a third portion of the compressed refrigerant to form a second expanded refrigerant stream, wherein the second expanded refrigerant stream is formed at a higher temperature than the first expanded refrigerant stream or the natural gas vapor stream;
wherein step (d) comprises warming the natural gas vapor stream, the first expanded refrigerant stream and the second expanded refrigerant stream in one or more heat exchanger sections, compressing the resulting warmed streams, and combining said streams to form a compressed refrigerant, wherein the natural gas vapor stream, first expanded refrigerant stream and second expanded refrigerant stream are combined prior to, during or after being warmed and compressed; and
wherein step (e) comprises cooling the first portion of the compressed refrigerant and the second portion of the compressed refrigerant, via indirect heat exchange with the natural gas vapor stream, the first expanded refrigerant stream and the second expanded refrigerant stream that are being warmed in step (d), to form respectively the first cold refrigerant stream and the second cold refrigerant stream, the first and second portions of the compressed refrigerant being cooled against the natural gas vapor stream, the first expanded refrigerant stream and the second expanded refrigerant stream, and the first portion of the compressed refrigerant being then further cooled against the natural gas vapor stream and the first expanded refrigerant stream such that the first cold refrigerant stream is formed at a lower temperature than the second cold refrigerant stream.
10. The method of claim 1 , wherein the second cold refrigerant stream is expanded in an expander portion of a compander having a compressor portion that is used for compressing at least a portion of the natural gas vapor stream and/or first expanded refrigerant stream in step (d).
11. The method of claim 1 , wherein in step (f) the resulting expanded streams are separated into vapor and liquid phases in one or more phase separators.
12. The method of claim 1 , wherein the method further comprises the step of:
(j) further cooling at least a portion of the first liquefied natural gas stream to form a liquefied natural gas product stream.
13. The method of claim 12 , wherein step (j) comprises flashing at least a portion of the first liquefied natural gas stream to form the liquefied natural gas product stream and one or more flash gas streams.
14. The method of claim 13 , wherein the method further comprises the step of:
(k) cooling and liquefying a fourth portion of the of the compressed refrigerant via indirect heat exchange with the one or more flash gas streams to form a second liquefied natural gas stream or set of liquefied natural gas streams; and
wherein step (j) comprises flashing the at least a portion of the first liquefied natural gas stream and the second liquefied natural gas stream or set of liquefied natural gas streams to form the liquefied natural gas product stream and the one or more flash gas streams.
15. The method of claim 14 , wherein the method further comprises the step of:
(l) cooling a fifth portion of the compressed refrigerant via indirect heat exchange with the one or more flash gas streams and then combining the fifth portion of the compressed refrigerant with the first portion of the compressed refrigerant during the cooling of the at least a first portion of the compressed refrigerant in step (e) to form the first cold refrigerant stream.
16. The method of claim 13 , wherein the method further comprises the step of:
(m) compressing the one or more flash gas streams to form a compressed flash gas stream, and cooling and liquefying the compressed flash gas stream, via indirect heat exchange with the natural gas vapor stream and the first expanded refrigerant stream that are being warmed in step (d), to form a third liquefied natural gas stream; and
wherein step (j) comprises flashing the at least a portion of the first liquefied natural gas stream and the third liquefied natural gas stream to form the liquefied natural gas product stream and the one or more flash gas streams.
17. The method of claim 13 , wherein the method further comprises the step of:
(m) compressing and combining the one or more flash gas streams with the natural gas vapor stream and the first expanded refrigerant stream to form the compressed refrigerant.
18. A system for removing natural gas liquids from and liquefying a natural gas feed stream, the system comprising:
one or more expansion devices and/or heat exchanger sections arranged and configured to expanding and/or cool a natural gas feed stream to form an expanded and/or cooled natural gas feed stream;
a distillation column having one or more separation sections, the distillation column being arranged and configured to receive the expanded and/or cooled natural gas feed stream into the distillation column below at least one of said separation sections and separate expanded and/or cooled natural gas feed stream into a natural gas liquids stream withdrawn from the bottom of the distillation column and a natural gas vapor stream withdrawn from the top of the distillation column;
one or more conduits, heat exchanger sections and compression stages arranged and configured to receive and warm the natural gas vapor stream and a first expanded refrigerant stream, compress the resulting warmed streams and combine said streams to form a compressed refrigerant, wherein the one or more conduits, heat exchanger sections and compression stages and are arranged and configured such that the natural gas vapor stream and first expanded refrigerant stream are combined prior to, during or after being warmed and compressed;
one or more conduits arranged and configured to pass a first portion of the compressed refrigerant and a second portion of the compressed refrigerant through the one or more heat exchanger sections so as to cool the first portion of the compressed refrigerant and the second portion of the compressed refrigerant via indirect heat exchange with the natural gas vapor stream and the first expanded refrigerant stream, to form respectively a first cold refrigerant stream and a second cold refrigerant stream, the first and second portions of the compressed refrigerant being cooled against the natural gas vapor stream and the first expanded refrigerant stream and the first portion of the compressed refrigerant being then further cooled against the natural gas vapor stream and the first expanded refrigerant stream such that the first cold refrigerant stream is formed at a lower temperature than the second cold refrigerant stream;
one or more expansion and separation devices arranged and configured to expand the first cold refrigerant stream, expand the second cold refrigerant stream, and combine and separate the resulting expanded streams into vapor and liquid phases to form a first liquefied natural gas stream from the liquid phase and the first expanded refrigerant stream from the vapor phase; and
one or more conduits and expansion devices arranged and configured to receive a reflux stream and expand and introduce the reflux stream into the top of the distillation column to provide reflux to the distillation column, wherein the reflux stream is formed from a portion of the first liquefied natural gas stream, a portion of the liquid phase separated in step (f), a portion of the first cold refrigerant stream withdrawn from said stream prior to said stream being separated in step (f), a further portion of the compressed refrigerant that has been cooled via indirect heat exchange with the natural gas vapor stream and the first expanded refrigerant stream that are being warmed in step (d), and/or a portion of a liquefied natural gas stream or liquefied natural gas product derived from the first liquefied natural gas stream.