SYSTEMS AND METHODS FOR REMOVAL OF NITROGEN FROM LNG

Abstract

A system for the removal of nitrogen from a liquid natural gas (LNG) stream. The system. comprises a feed heat changer and a stripper column. The heat receives the LNG stream and cools the LNG stream via heat exchange with a stripper column side-draw stream to yield a cooled LNG stream and a heated side-draw stream. The stripper column receives the cooled LNG stream at a first tray and the heated side-draw stream. The stripper column produces the stripper column side-draw stream, a stripper column overhead stream, and a stripper column bottom stream. The stripper column side-draw stream is taken from the stripper column at a second tray. The second tray is at least about 15 feet higher than the feed heat exchanger.

Claims

1. A system for the removal of nitrogen from a liquid natural gas (LNG) stream, the system comprising: a feed heat exchanger, wherein the heat is configured to receive the LNG stream and to cool the LNG stream via heat exchange with a stripper column side-draw stream to yield a cooled LNG stream and a heated side-draw stream; and a stripper column, wherein the stripper column is configured to receive the cooled LNG stream at a first tray and the heated side-draw stream, and wherein the stripper column is configured to produce the stripper column side-draw stream, a stripper column overhead stream, and a stripper column bottom stream, wherein the stripper column is configured such that the stripper column side-draw stream is taken from the stripper column at a second tray, wherein the second tray is at least about 15 feet higher than the feed heat exchanger.

2. The system of claim 1, further comprising a pressure reduction device, wherein the pressure reduction device is configured to reduce the pressure of the stripper column side-draw stream prior to introduction into the feed heat exchanger.

3. The system of claim 2, wherein the pressure reduction device is a valve.

4. The system of claim 2, further comprising a control device, wherein the control device is configured to control the flow of the heated side-draw stream prior to introduction into the feed heat exchanger

5. The system of claim 4, wherein the control device is a flow control valve.

6. The system of claim 1, wherein the system is configured to receive the LNG stream from a LNG liquefaction plant.

7. The system of claim 1, wherein the LNG stream is characterized as a relatively-high nitrogen content LNG stream.

8. The system of claim 7, wherein the stripper column bottom stream has less than about 0.9 mole % nitrogen.

9. The system of claim 1, wherein the cooled LNG stream has a temperature of from about 256° F. to −275° F.

10. The system of claim 1, wherein the second tray is at least about 20 feet higher than the feed heat exchanger.

11. A method for the removal of nitrogen from a liquid natural gas (LNG) stream, the method comprising: receiving, at a feed heat exchanger, the LNG stream; cooling the LNG stream via heat exchange with a stripper column side-draw stream to yield a cooled LNG stream and a heated side-draw stream; receiving, at a stripper column, the cooled LNG stream at a first tray and the heated side-draw stream; producing the stripper column side-draw stream from the stripper column at a second tray that is at least about 15 feet higher than the feed heat exchanger; and producing a stripper column overhead stream and a stripper column bottom stream.

12. The method of claim 11, further comprising controlling the flow of the stripper column side-draw stream prior to introducing the stripper column side-draw stream into the feed heat exchanger.

13. The method of claim 12, wherein controlling the flow of the stripper column side-draw stream comprises passing the stripper column side-draw stream through a first valve.

14. The method of claim 11, further comprising controlling the flow of the heated side-draw stream prior to introducing the heated side-draw stream into the feed heat exchanger.

15. The method of claim 14, wherein controlling the flow of the heated side-draw stream comprising passing the heated side-draw stream through a second valve.

16. The method of claim 11, wherein receiving the LNG stream comprises receiving the LNG stream from a LNG liquefaction plant.

17. The method of claim 11, wherein the LNG stream is characterized as a relatively-high nitrogen content LNG stream.

18. The method of claim 17, wherein the s column bottom stream has less than about 0.9 mole % nitrogen.

19. The method of claim 11, wherein cooling the LNG stream comprises cooling the LNG stream to a temperature of from about −256° F. to −275° F.

20. The method of claim 11, wherein the second tray is at least about 20 feet higher than the feed heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is an embodiment of the nitrogen removal system.

[0019] FIG. 2 illustrates the heat exchange curves for the feed heat exchanger of FIG. 1.

[0020] FIG. 3 depicts the nitrogen removed via operation of the system of FIG. 1.

DETAILED DESCRIPTION

[0021] Disclosed herein are systems and methods for removing nitrogen from a LNG product stream using a specially-configured stripper and feed heat exchanger. In some of the disclosed embodiments, the removal of nitrogen from the LNG product stream is accomplished without the use of external heating and cooling, for example, thereby increasing the heating value of the resultant LNG product stream.

[0022] Referring to FIG. 1, an embodiment of the disclosed nitrogen removal (NR) system 100 is illustrated. In the embodiment of FIG. 1, the NR system 100 generally comprises a feed heat exchanger 56, a first valve 57, a second valve 58, and a stripper column 59.

[0023] In the embodiment of FIG. 1, the NR system 100 is shown in operation. In operation, a LNG feed liquid stream 4 is introduced into the feed heat exchanger 56. The LNG feed liquid stream 4 may be from an LNG liquefaction plant, for example, of which the NR system may be a sub-system.

[0024] In such an embodiment, a gas stream, which may have a pressure of about 850 psig, may be treated in an acid gas removal unit to decrease the acid gas content thereof, for example, such that the gas stream contains no more than 50 ppmv CO.sub.2 and 4 ppmv H.sub.2S. The treated gas stream may be dried, for example, in a molecular sieve dehydration unit to thereby produce a dry gas stream. If mercury is present in the feed gas, the mercury may also be removed, for example, to lessen the potential for mercury corrosion of any process equipment. This purified gas stream may then be fed to the LNG liquefaction plant, which produces the LNG feed stream 4 (e.g., a high-pressure subcooled LNG stream).

[0025] The LNG feed liquid stream 4 may have a temperature and pressure of about −238° F. and about 850 psig; for example, the LNG feed liquid stream 4 may be characterized as a high-pressure LNG stream. The LNG liquid gas stream 4 may be characterized as a relatively-high nitrogen content LNG stream, for example, having about 4% nitrogen content, alternatively, about 5%, alternatively, about 6%, alternatively, about 7%, alternatively, about 8%, alternatively, about 9%, alternatively, about 10%, alternatively, about 11%, alternatively, about 12%, alternatively, about 13%, alternatively, about 14%, alternatively, up to about 15 mole % nitrogen content.

[0026] The LNG feed liquid stream 4 (e.g., a high-pressure LNG stream) is cooled in the feed heat exchanger 56 by a letdown stripper column side-draw stream 10 to yield a subcooled LNG stream 7. The subcooled LNG stream 7 may have a temperature of from about −256° F. to −275° F.

[0027] Referring to FIG. 2, the heat exchange curves for the feed heat exchanger 56 are shown. In an embodiment, the feed heat exchanger 56 is configured to operate with a close temperature approach, for example, thereby minimizing thermodynamic losses. In an embodiment, the feed heat exchanger 56 is a brazed aluminum exchanger, which can achieve close temperature approaches. Not intending to be bound by theory, as can been seen in FIG. 2, the pinch point of the heat exchange curves is in the mid-section where vaporization of the side-draw liquid occurs.

[0028] Referring again to FIG. 1, the subcooled LNG stream 7 is passed through the second valve 58 (e.g., a JT valve), for example, to reduce (e.g., decrease or “let down”) the pressure of the subcooled LNG stream 7 and thereby yield a two-phase stream 8. In an embodiment, passing the subcooled LNG stream 7 through the second valve 58 to thereby form the two-phase stream 8 may further cool or subcool the LNG, for example, to a temperature of about −262° F. to −287° F. Not intending to be bound by theory, passing the subcooled LNG stream 7 through the second valve 58 may help to suppress the temperature thereof, for example, to suppress the temperature of the subcooled LNG stream 7 to a preferred lower temperature, for example, about −238° F. to −275° F. In alternative embodiments, any suitable pressure reduction device may be employed in place of the second valve 58.

[0029] in the embodiment of FIG. 1, the two-phase stream 8 is fed to the stripper column 59, which may comprise a plurality of trays. In the embodiment of FIG. 1, the two-phase stream 8 is fed into the stripper column 59 at a first tray, for example, a two-phase distributor tray 60 configured to separate, alternatively, to substantially separate, the two phases of the two-phase stream 8, for example, to separate the flash vapor component (e.g., BOG) from the liquid component. In an embodiment, the two-phase stream 8 is fed into the stripper column 59 at an upper section of the stripper column 59; for example, the first tray to which the two-phase stream 8 is fed may be in an upper (e.g., an upper-most) section of the stripper column.

[0030] In the embodiment, the stripper column 59 is operated to yield a stripper column side-draw stream 9, a stripper column overhead vapor stream 6, and a stripper column bottom stream 5. The stripper column side-draw stream 9 is withdrawn from a second tray of the stripper column, for example, a chimney tray 61. In an embodiment, the second tray (e.g., the chimney tray) is configured to draw a liquid from the stripper column 59. In an embodiment, the chimney is the second upper-most tray of the stripper column 59, alternatively, the third upper-most tray of the stripper column 59, alternatively, the fourth or, alternatively, a higher upper-most tray of the stripper column 59. Additionally or alternatively, in an embodiment, the tray from which the stripper column side-draw stream 9 is withdrawn is at least about 15 feet higher than the feed heat exchanger 56, alternatively, at least about 16 feet, alternatively, at least about 17 feet, alternatively, at least about 18 feet, alternatively, at least about 19 feet, alternatively, at least about 20 feet, alternatively, at least about 22 feet, alternatively, at least about 24 feet higher than the feed heat exchanger. Not intending to be bound by theory, the elevation at which the stripper column side-draw stream 9 is withdrawn may be sufficient to provide sufficient head to operate the thermosiphon system hydraulic.

[0031] Referring again to FIG. 1, the stripper column side-draw stream 9 is passed through the first valve 57, for example, to control flow rate of the stripper column side-draw stream 9 while maintaining the level in the chimney tray 61 for stability, and thereby yield the letdown stripper column side-draw stream 10. In alternative embodiments, any suitable controlling device may be employed in place of the first valve 57. The letdown stripper column side-draw stream 10 may have a temperature of from about −262 to −280° F.

[0032] In the embodiment of FIG. 1, the letdown stripper column side-draw stream 10 is introduced into the feed heat exchanger 56, for example, to cool the LNG feed liquid stream 4 as discussed herein, thereby forming a heated LNG stream 11. The heated LNG stream 11 may be characterized as a two-phase stream.

[0033] In the embodiment of FIG. 1, the heated LNG stream 11 is fed (e.g., returned) to the stripper column 59. In an embodiment, the heated LNG stream 11 is fed to the second lower-most tray, alternatively, the third lower-most tray, alternatively, the fourth lower-most tray or, alternatively, a higher tray above the bottom section of the stripper column 59. Not intending to be bound by theory, introducing the heated LNG stream 11 to such a relatively-lower tray may ensure sufficient contact and mixing with the various tray liquids of the stripper column.

[0034] The stripper column overhead vapor stream 6 produced by the stripper column may be characterized as having a nitrogen content of from about 35 mole % to 80 mole %. The stripper column bottom stream 5 may be characterized as having a nitrogen content of from about 0.2 to 0.9 mole %. For example, the LNG yielded via the stripper column bottom stream 5 may be suitable for LNG export or storage. The high nitrogen content pertains to LNG feed stream with 15 mole % while the low nitrogen content pertains to the 4 mole % LNG feed.

[0035] Referring to FIG. 3, the nitrogen removal resulting from the operation of the system of FIG. 1 is shown. As shown in FIG. 3, when the stream entering the process (e.g., LNG feed liquid stream 4) has a nitrogen content of about 4 mole %, the disclosed system and process can produce an LNG product stream (e.g., the stripper column bottom stream 5) with about 0.2 mole % nitrogen, which is significantly lower than would have been achieved via conventional systems and methods utilizing a flash-drum at the end of such conventional process, which would be about 1 mole %. As also shown in FIG. 3, when the stream entering the process (e.g., LNG feed gas stream 4) has a nitrogen content of about 15 mole %, where a conventional end-of-process flash-drum is unable to remove a sufficient amount of nitrogen, the disclosed system and process can produce an LNG with less than 0.9 mole % nitrogen.

[0036] Further, in addition to yielding an LNG product meeting desired LNG specifications, the disclosed systems and processes also yield an LNG product having an improved heating value, for example, the heating value of the LNG product may be improved by from about 5 to 17%, depending on the nitrogen content of the LNG feed. As such, the disclosed systems and processes are particularly useful for processing natural gas in existing facilities, for example, in order to meet nitrogen specifications, from fields yielding natural gas with an increasing nitrogen content. As such, the disclosed systems and processes are cost-effective and are capable of achieving nitrogen removal from an LNG stream, maximizing the heating value of such LNG stream, increasing the value of the LNG product.

Additional Embodiments

[0037] A first embodiment, which is a system for the removal of nitrogen from a liquid natural gas (LNG) stream, the system comprising a feed heat exchanger, wherein the heat is configured to receive the LNG stream and to cool the LNG stream via heat exchange with a stripper column side-draw stream to yield a cooled LNG stream and a heated side-draw stream; and a stripper column, wherein the stripper column is configured to receive the cooled LNG stream at a first tray and the heated side-draw stream, and wherein the stripper column is configured to produce the stripper column side-draw stream, a stripper column overhead stream, and a stripper column bottom stream, wherein the stripper column is configured such that the stripper column side-draw stream is taken from the stripper column at a second tray, wherein the second tray is at least about 15 feet higher than the feed heat exchanger.

[0038] A second embodiment, which is the system of the first embodiment, further comprising a pressure reduction device, wherein the pressure reduction device is configured to reduce the pressure of the stripper column side-draw stream prior to introduction into the feed heat exchanger.

[0039] A third embodiment, which is the system of the second embodiment, wherein the pressure reduction device is a valve.

[0040] A fourth embodiment, which is the system of one of the second through the third embodiments, further comprising a control device, wherein the control device is configured to control the flow of the heated side-draw stream prior to introduction into the feed heat exchanger.

[0041] A fifth embodiment, which is the system of the fourth embodiment, wherein the control device is a flow control valve.

[0042] A sixth embodiment, which is the system of one of the first through the fifth embodiments, wherein the system is configured to receive the LNG stream from a LNG liquefaction plant.

[0043] A seventh embodiment, which is the system of one of the first through the sixth embodiments, wherein the LNG stream is characterized as a relatively-high nitrogen content LNG stream.

[0044] An eighth embodiment, which is the system of the seventh embodiment, wherein the stripper column bottom stream has less than about 0.9 mole % nitrogen.

[0045] A ninth embodiment, which is the system of one of the first through the eighth embodiments, wherein the cooled LNG stream has a temperature of from about −256° F. to −275° F.

[0046] A tenth embodiment, which is the system of one of the first through the ninth embodiments, wherein the second tray is at least about 20 feet higher than the feed heat exchanger.

[0047] An eleventh embodiment, which is a method for the removal of nitrogen from a liquid natural gas (LNG) stream, the method comprising receiving, at a feed heat exchanger, the LNG stream, cooling the LNG stream via heat exchange with a stripper column side-draw stream to yield a cooled LNG stream and a heated side-draw stream, receiving, at a stripper column, the cooled LNG stream at a first tray and the heated side-draw stream, producing the stripper column side-draw stream from the stripper column at a second tray that is at least about 15 feet higher than the feed heat exchanger; and producing a stripper column overhead stream and a stripper column bottom stream.

[0048] A twelfth embodiment, which is the method of the eleventh embodiment, further comprising controlling the flow of the stripper column side-draw stream prior to introducing the stripper column side-draw stream into the feed heat exchanger.

[0049] A thirteenth embodiment, which is the method of the twelfth embodiment, wherein controlling the flow of the stripper column side-draw stream comprises passing the stripper column side-draw stream through a first valve.

[0050] A fourteenth embodiment, which is the method of one of the eleventh through the thirteenth embodiments, further comprising controlling the flow of the heated side-draw stream prior to introducing the heated side-draw stream into the feed heat exchanger.

[0051] A fifteenth embodiment, which is the method of the fourteenth embodiment, wherein controlling the flow of the heated side-draw stream comprising passing the heated side-draw stream through a second valve.

[0052] A sixteenth embodiment, which is the method of one of the eleventh through the fifteenth embodiments, wherein receiving the LNG stream comprises receiving the LNG stream from a LNG liquefaction plant.

[0053] A seventeenth embodiment, which is the method of one of the eleventh through the sixteenth embodiments, wherein the LNG stream is characterized as a relatively-high nitrogen content LNG stream.

[0054] An eighteenth embodiment, which is the method of the seventeenth embodiment, wherein the stripper column bottom stream has less than about 0.9 mole % nitrogen.

[0055] A nineteenth embodiment, which is the method of one of the eleventh through the eighteenth embodiments, wherein cooling the LNG stream comprises cooling the LNG stream to a temperature of from about −256° F. to −27.5° F.

[0056] A twentieth embodiment, which is the method of one of the eleventh through the nineteenth embodiments, wherein the second tray is at least about 20 feet higher than the feed heat exchanger.