Method for recovery of natural gas liquids for liquefied natural gas

Abstract

The invention includes a process and apparatus for separating a liquefied natural gas (LNG) stream containing methane and lighter components and heavy hydrocarbon components into a more volatile gas fraction containing a substantial amount of the methane and lighter components and a less volatile fraction containing a large portion of the heavy hydrocarbon components. The process includes splitting the LNG stream into two feed streams, preheating the resulting first feed stream and providing the feed stream to a fractionation tower at two locations.

Description

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to the recovery of natural gas liquids (NGL) from liquefied natural gas (LNG) streams.

2. Description of the Related Art

Many attempts have been made to recover NGL from LNG streams. One such example can be found in U.S. Pat. No. 6,564,579 issued to McCartney titled Method for Vaporizing and Recovery of Natural Gas Liquids from Liquefied Natural Gas Streams (hereinafter “McCartney”). McCartney describes a process, as shown in FIG. 4 of the present application, that uses of an overhead compressor to increase the bubble point of a lean gas stream exiting a fractionation tower 10. In this prior art process, a rich low pressure LNG stream 1 is pumped in pump 2 to 450 psia. LNG stream 3 exiting pump 2 is heated in exchanger 5 to −132.5° F. to produce a heated LNG stream 6. Heated LNG stream 6 is introduced to fractionation tower 10 as a top tower feed stream.

Fractionation tower 10 is a distillation tower that produces a tower bottoms stream 12 and a tower overhead stream 14. Tower bottoms stream 12 is sent to a tower reboiler 13 that is used to maintain an amount of methane in an NGL product stream 12. A vapor return stream from reboiler 13 is sent to fractionation tower 10, while NGL product is drawn off as NGL product stream 12. Tower overhead stream 14 is compressed in lean gas compressor 27 to 500 psia to produce a partially boosted stream 28. Partially boosted stream 28 is cooled to −137° F. and completely condensed in first exchanger 5 to produce a low pressure lean LNG stream 21. Low pressure lean LNG stream is then pumped by pump 22 to elevate its pressure to 1422 psia to produce a high pressure lean LNG product stream 23, which is the high pressure lean LNG product that is sent for vaporization and/or energy recovery.

As can be seen in Table II, recovery of LNG products is limited in the prior art. Simulation results of the embodiments of the current invention compared to alternate schemes show that for lower recoveries and NGL production, there is an increase in capital cost due to the addition of a compressor and increased heat exchanger area for prior art. Utility consumption is also affected.

Co-pending U.S. patent application Ser. No. 10/651,178 titled Optimized Heating Value in Natural Gas Liquids Recovery Scheme illustrates another attempt at recovering NGL products from LNG streams. FIG. 2 illustrates one scheme that uses subcooled rich LNG to increase NGL recovery. In this related process, rich low pressure LNG stream 1 is pumped in pump 2 to about 525 psia to produce LNG stream 3. LNG stream 3 is then split into two streams, a first feed stream 4 and a second feed stream 24. First feed stream 4, which is the larger of the two streams and contains about 93% of LNG stream 3, is heated in first exchanger 5 to −132.5° F. to produce a heated first feed stream 6. Heated first feed stream 6, which is still all liquid, is introduced to fractionation tower 10 as a tower bottom feed stream. Second feed stream 24 is sent as a top feed stream to fractionation tower 10.

As in the prior art process shown in FIG. 4, fractionation tower 10 is a distillation tower that produces a tower overhead stream 14 and a tower bottom stream 12. Tower bottom stream 12 is sent to a tower reboiler 13 that is used to maintain an amount of methane in NGL product stream 12. Reboiler vapor stream returns to fractionation tower 10, while NGL product is drawn off of reboiler 13 as NGL product stream 12.

Tower overhead stream 14 is cooled to −133.7° F. and completely condensed in first exchanger 5 to produce low pressure lean LNG stream 21. Low pressure lean LNG stream 21 is pumped in pump 22 to elevate its pressure to 1422 psia to produce high pressure lean LNG product stream 23, which is the high pressure lean LNG product that is sent for vaporization and/or energy recovery.

SUMMARY OF THE INVENTION

The present invention includes a process and apparatus to increase the recovery of NGL. As an embodiment of the present invention, a process is provided for separating a liquefied natural gas (LNG) stream into a more volatile fraction containing a substantial amount of methane and lighter components and a less volatile fraction containing a large portion of heavy hydrocarbon components is advantageously provided. In this embodiment, the LNG stream is split into a first feed stream and a second feed stream. At least a portion of the first feed stream is preheated and supplied to a fractionation tower as a first tower feed stream.

In preferred embodiments of the present invention, fractionation tower produces a tower overhead stream and a tower bottoms stream. Tower overhead stream preferably includes the more volatile fraction of the LNG stream that contains a substantial amount of methane and lighter components. Tower bottoms stream preferably includes the less volatile fraction of the LNG stream that contains the heavy hydrocarbon components.

At least a portion of the second feed stream is heated and supplied to the fractionation tower as a second tower feed stream. At least a portion of the tower overhead stream is cooled and partially condensed to produce a partially condensed tower overhead stream. Partially condensed tower overhead stream is separated into a lean vapor stream and a tower reflux stream being sent to the fractionation tower. Lean vapor stream is subcooled so that the lean vapor stream is substantially condensed thereby producing a lean LNG stream. Lean LNG stream is then pumped to a high pressure.

In another embodiment of the invention, the liquefied natural gas (LNG) stream is split into a first feed stream and a second feed stream. The feed streams define, respectively, a first feed stream enthalpy and a second feed stream enthalpy. At least a portion of the first feed stream is preheated and provided to the fractionation tower as the first tower feed stream. From the fractionation tower, the tower overhead stream is produced containing the more volatile fraction. The tower bottoms stream containing the less volatile fraction is also produced from the fractionation tower. The second feed stream is supplied to the fractionation tower such that the first feed stream and the second tower feed stream have a substantially common composition but the first tower feed stream enthalpy differs from the second feed stream enthalpy. The tower overhead stream is compressed to produce a compressed overhead stream. The compressed overhead stream is cooled until it is at least substantially condensed thereby producing a lean LNG stream. In one preferred embodiment, the compressed overhead stream is cooled until it is subcooled. The lean LNG stream is pumped to a higher pressure thereby creating a high pressure lean LNG stream.

In yet another embodiment of the invention, the process for separating the liquefied natural (LNG) stream includes preheating at least a portion of the LNG stream to produce first tower feed stream. The first tower feed stream is supplied to the fractionation tower that produces the tower overhead stream and tower bottoms stream. At least a portion of the tower overhead stream is cooled so that the portion of the tower overhead stream is at least substantially condensed thereby producing the lean LNG stream. The tower overhead stream can be subcooled. The lean LNG stream is pumped to a higher pressure to produce a high pressure lean LNG stream. The high pressure lean LNG stream is split into a lean tower reflux stream and a lean LNG product stream. The lean tower reflux stream is cooled to produce a cooled lean tower reflux stream and the cooled lean tower reflux stream is supplied to the fractionation tower.

Yet another preferred embodiment includes a process for separating the liquefied natural gas (LNG) stream including splitting the LNG stream into first feed stream and second feed stream. The second feed stream is supplied to the fractionation tower as second tower feed stream to produce the tower overhead stream and the tower bottoms stream 12 including the less volatile fraction containing the heavy hydrocarbon components. At least a portion of the first feed stream is preheated to produce the first tower feed stream. The first tower feed stream is supplied to the fractionation tower. The tower overhead stream is compressed to produce compressed overhead stream. The compressed overhead stream is cooled such that the compressed overhead stream is substantially condensed thereby producing lean LNG stream. The lean LNG stream is pumped to a higher pressure to produce high pressure lean LNG stream. The high pressure lean LNG stream is split into lean tower reflux stream and lean LNG product stream. The lean tower reflux stream is cooled to produce cooled lean tower reflux stream that is supplied to the fractionation tower.

Yet another embodiment of the invention includes a process for separating a liquefied natural gas (LNG) stream including preheating at least a portion of the LNG stream and supplying the at least a portion of the LNG stream to fractionation tower as first tower feed stream. The fractionation tower produces tower overhead stream and tower bottoms stream. At least a portion of the tower overhead stream is expanded to a lower pressure such that the at least a portion of the tower overhead stream is partially condensed to produce a partially condensed low pressure vapor stream. The partially condensed low pressure vapor stream is separated into a lean vapor stream and tower reflux stream 18. The lean vapor stream is compressed and cooled to create lean LNG stream. The tower reflux stream is cooled thereby producing a cooled lean tower reflux stream and the lean LNG stream is pumped to a higher pressure creating high pressure lean LNG stream.

In yet another embodiment of the present invention, the process for separating a liquefied natural gas (LNG) stream includes preheating at least a portion of the LNG stream to produce first tower feed and supplying the first tower feed to fractionation tower. From the fractionation tower is produced tower overhead stream and tower bottoms stream. At least a portion of the tower overhead stream is cooled and thereby partially condensing to produce partially condensed tower overhead stream. The partially condensed tower overhead stream is separated into lean vapor stream and lean LNG stream. The lean LNG stream is pumped to a high pressure to produce high pressure lean LNG stream. The high pressure lean LNG stream is split into a lean tower reflux stream 25 and lean LNG product stream. The lean tower reflux stream is cooled to produce cooled lean tower reflux stream. The cooled lean tower reflux stream is supplied to the fractionation tower. The lean vapor stream is compressed to high pressure to produce compressed second lean vapor stream. The compressed second lean vapor stream is combined with the lean LNG product stream.

In another preferred embodiment, the invention includes an apparatus for separating a liquefied natural gas (LNG) stream, the apparatus including means for splitting the LNG stream into a first feed stream and a second feed stream. Also included is a first exchanger operable to preheat the first feed stream to provide a first tower feed stream. A fractionation tower is included for receiving the first tower feed stream and to produce tower overhead stream and tower bottoms stream. A first cooler is provided that is operable to cool and partially condense at least a portion of the tower overhead stream to produce a partially condensed tower overhead stream. The first cooler is operable to allow for heat exchange between the portion of the tower overhead stream and the second feed stream. A separator is operable to separate the partially condensed overhead tower stream into lean vapor stream and tower reflux stream, the tower reflux stream returning to the fractionation tower. The first exchanger is operable to allow for heat exchange between the lean vapor stream and the first feed stream, the lean vapor stream being cooled in the first exchanger to provide lean LNG stream. A pump operable for pumping the lean LNG stream to high pressure LNG stream is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, may be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof that is illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and are therefore not to be considered limiting of the invention's scope as it may admit to other equally effective embodiments.

FIG. 1 is a simplified flow diagram of a hydrocarbon recovery process that is configured for increased recovery of heavy components from an inlet LNG stream through the use of a split feed stream and lean tower reflux stream in accordance with an embodiment of the present invention;

FIG. 2 is a simplified flow diagram of a hydrocarbon recovery process that is configured for increased recovery of heavy components from an inlet LNG stream through the use of a split feed stream in accordance with an embodiment of a process described in co-pending U.S. patent application Ser. No. 10/651,178 titled Optimized Heating Value in Natural Gas Liquids Recovery Scheme;

FIG. 3 is a simplified flow diagram of a hydrocarbon recovery process that is configured for increased recovery of heavy components from an inlet LNG stream through the use of a lean LNG reflux stream in accordance with an embodiment of the present invention;

FIG. 4 is a simplified flow diagram of a prior art process as shown in U.S. Pat. No. 6,564,579 issued to McCartney, which illustrates a typical system and process for vaporizing LNG and separating natural gas liquids from the LNG stream in accordance with prior art;

FIG. 5 is a simplified flow diagram of a hydrocarbon recovery process that is configured for increased recovery of heavy components from an inlet LNG stream through the use of an overhead compressor in accordance with an embodiment of the present invention.

FIG. 6 is a simplified flow diagram of a hydrocarbon recovery process that is configured for increased recovery of heavy components from an inlet LNG stream through the use of an overhead compressor and a lean LNG reflux stream in accordance with an embodiment of the present invention.

FIG. 7 is a simplified flow diagram of a hydrocarbon recovery process that is configured for increased recovery of heavy components from an inlet LNG stream through the use of an overhead condenser and an overhead compressor in accordance with an embodiment of the present invention.

FIG. 8 is a simplified flow diagram of a hydrocarbon recovery process that is configured for increased recovery of heavy components from an inlet LNG stream through the use of an overhead expander reflux stream in accordance with an embodiment of the present invention.

FIG. 9 is a simplified flow diagram of a hydrocarbon recovery process that is configured for increased recovery of heavy components from an inlet LNG stream through the use of a partial flow compressor in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the description of the figures, the same numbers are used to refer to similar or same components in all figures. For clarity and simplicity, not all equipment is shown as they are considered known to those skilled in the art. For consistency, the same equipment parameters such as number of trays, efficiencies, pressure drops, product specifications etc have been used to compare processes.

Embodiments of the present invention have been compared with prior art processes to illustrate improvements in either yield or power consumptions. The processes described herein were compared using the inlet LNG stream composition and conditions shown in Table I.

	TABLE I
	Component	Units
	Nitrogen	Mol %	0.46
	Methane	Mol %	89.79
	Ethane	Mol %	6.47
	Propane	Mol %	2.23
	i-Butane	Mol %	.42
	n-Butane	Mol %	.63
	Temperature	F	−241.6
	Pressure	Psia	71.6
	Flow	MMTPA	5

In all embodiments of the process, liquefied natural gas (LNG) stream 3 contains methane and lighter components and heavy hydrocarbon components. In order to recover NGL products, LNG stream 3 is separated into a more volatile fraction that contains a substantial amount of the methane and lighter components and a less volatile fraction that contains a large potion of the heavy hydrocarbon components.

Referring to the drawings, FIG. 1 shows a process scheme embodiment of the present invention that is used to recover NGL products. LNG inlet stream 1 is pumped by LNG pump 2 to about 525 psia. LNG stream 3 exiting LNG pump 2 is split into two streams, first feed stream 4 and second feed stream 7. First feed stream 4, which is typically the larger of the two streams and contains about 95% of LNG stream 3, is preheated in first heater or first exchanger 5 to about −133.5° F. to provide first tower feed stream 6. First tower feed stream 6, which is still substantially all liquid, is introduced into fractionation or distillation tower 10. Second feed stream 7 is heated in first cooler 8 to about −132.9° F. to produce second tower feed stream 9, which is introduced into the fractionation tower 10. In preferred embodiments of the present invention, fractionation tower 10 produces a tower overhead stream 14 and a tower bottoms stream 12. Tower overhead stream 14 contains the more volatile fraction containing a substantial amount of methane contained within LNG stream 3. Tower bottoms stream 12 contains the less volatile fraction containing the heavy hydrocarbon component. In a preferred embodiment, bottom liquid stream 11 is sent to reboiler 13 to produce tower bottoms stream 12. Vapor stream from tower reboiler 13 returns to fractionation tower 10. In this embodiment, the reboiler can be used maintain methane content in a NGL product stream 12. Alternate embodiments include substituting other heat sources known in the art to the bottom of the fractionation column 10.

Tower overhead stream 14 is cooled to about −128.5° F. in first cooler 8. This partially condenses at least a portion of the tower overhead stream 14 to produce a partially condensed tower overhead stream 15. Partially condensed tower overhead stream 15 is sent to reflux accumulator or separator 16. Partially condensed tower overhead stream 15 is separated into a lean vapor stream 17 and a tower reflux stream 18. Tower reflux stream 18 is sent to fractionation tower 10 preferably at a top tower feed location. Tower reflux stream 18 can be cooled prior to sending tower reflux stream 18 to fractionation tower 10. Tower reflux stream 18 is sent to fractionation tower 10 preferably by pumping with reflux pump 19.

Lean vapor or residue stream 17 is cooled to about −134.5° F. and substantially completely condensed in first exchanger 5 thereby producing lean LNG stream 21. Lean LNG stream 21 is then pumped preferably by pump 22 to elevate its pressure to about 1422 psia. High pressure lean LNG stream 23 leaving the pump is a product stream that can be sent for vaporization and/or energy recovery.

The process shown in FIG. 1 can be used when the heavy hydrocarbon components include C2 components, C3 components, and heavier components, i.e., ethane recovery. The process shown in FIG. 1 can also be used when the heavy hydrocarbon components include C3 components, and heavier components, i.e., propane recovery.

In embodiments of the present invention, the step of cooling at least a portion of tower overhead stream 14 includes cooling at least a portion of tower overhead stream 14 by heat exchange contact with at least a portion of the second feed stream 7 thereby providing at least a portion of duty for the step of preheating the at least a portion of the second feed stream. Heat exchange contact between second feed stream 7 and tower overhead stream 14 can be performed in first cooler 8. In alternate embodiments, such as demonstrated in FIG. 5, In embodiments of the present invention, the step of cooling lean vapor stream 17 includes cooling lean vapor stream 17 by heat exchange contact with at least a portion of first feed stream 4 in first exchanger 5 thereby providing at least a portion of the duty for the step of preheating at least a portion of first feed stream 4.

In another embodiment of the present invention, such as is demonstrated in FIG. 7, the embodiments described above in reference to FIG. 1 further include the step of compressing the lean vapor stream 17 prior to the step of cooling the lean vapor stream.

In embodiments of the present invention, the tower reflux stream 18 is cooled to create cooled tower reflux stream 26, which is then supplied to the fractionation tower. The cooled tower reflux stream 26 is preferably supplied to the fractionation tower at a top feed position. The tower reflux stream 18 is cooled by heat exchange contact with at least a portion of second feed stream 7 thereby providing at least a portion of the duty for the step of preheating at least a portion of second feed stream 7. In an alternate embodiment, the tower reflux stream 18 is cooled by heat exchange contact with at least a portion of the first feed stream 4 thereby providing at least a portion of the duty for the step of preheating at least a portion of the first feed stream 4.

In certain embodiments, the step of preheating at least a portion of the first feed stream 4 to create first tower feed stream 6 advantageously includes supplying the first tower feed stream 6 to the fractionation tower 10 as a tower bottom feed stream. As noted previously, various means of providing heat or energy to the bottom of the fractionation tower are encompassed. An alternate embodiment includes providing the first tower feed stream at a position in the column lower than the position where the second tower feed stream enters the fractionation tower 10. This advantageously provides a driving force to the separation in the fractionation tower 10.

As another embodiment of the present invention, FIG. 3 shows a scheme that uses cooled lean tower reflux stream 26 to increase NGL recovery in NGL product stream 12. In this embodiment, it is advantageous that cooled lean tower reflux stream 26 be subcooled. Rich low pressure LNG stream 1 is pumped in LNG pump 2 to about 525 psia. LNG stream 3 exiting LNG pump 2 is heated in first exchanger 5 to about −133.2° F. to produce first tower feed stream 6. First tower feed stream 6, which is still substantially all liquid, is introduced into fractionation tower 10 preferably as a bottom feed stream.

In this embodiment, tower overhead stream 14 is cooled to about −133.7° F. and is substantially completely condensed in first exchanger 5 to produce a low pressure lean LNG stream 21. Lean LNG stream 21 exiting exchanger 5 is elevated in pressure preferably by pump 22 to about 1422 psia to produce high pressure lean LNG stream 23. High pressure lean LNG stream 23 is then split into lean tower reflux stream 25 and lean LNG product stream 48. Lean LNG stream 48 can be sent for vaporization and/or energy recovery. Lean tower stream 25 is cooled in first exchanger 5 to about −233° F. and is then sent, preferably as a top feed stream, to fractionation tower 10. The process described above and its various alternate embodiments can be used when the heavy hydrocarbon components include C2 components, C3 components, and heavier components, i.e., ethane recovery. Similarly, the process described above and its various alternate embodiments can also be used when the heavy hydrocarbon components include C3 components, and heavier components, i.e., propane recovery.

In one embodiment shown in FIG. 3, the step of cooling at least a portion of the tower overhead stream 14 includes cooling the portion of the tower overhead stream by heat exchange contact with at least a portion of LNG stream 3 thereby providing at least a portion of the duty for the step of preheating the LNG stream 3. Alternately or in addition, lean tower reflux stream 25 is cooled by heat exchange contact with LNG stream 3 thereby providing at least a portion of the duty for the step of preheating the LNG stream 3. In one embodiment, the first tower feed stream 6 is introduced to the fractionation tower below cooled lean tower reflux stream 26 to provide additional driving force due to enthalpy differences. The cooled lean tower reflux stream 26 is preferably supplied to the fractionation tower as a tower top feed stream.

In certain instances, first exchanger 5 can act as a critical path element possibly limiting recoveries due to the need to produce a completely liquid lean LNG stream from the exchanger that can be pumped. Attempts to increase recoveries can result in exchanger pinch on first exchanger 5 and/or possibly result in a two phase lean LNG stream at the outlet of first exchanger 5, which would then require further processing. Regarding FIG. 5, lean gas compressor 27 is introduced to avoid such a pinch and to avoid a two phase stream at the first exchanger 5 outlet. The bubble point of the lean residue gas stream leaving the tower is increased in this manner. Tower overhead stream 14 can be compressed, and then condensed at a higher temperature.

As another embodiment of the present invention, FIG. 5 shows another scheme that can be used to separate LNG stream 3 into the more volatile fraction containing a substantial amount of the methane and lighter components and a less volatile fraction containing a large portion of the heavy hydrocarbon components. In this embodiment, LNG inlet stream 1 is pumped by LNG pump 2 to about 475 psia. LNG stream 3 exiting LNG pump 2 is split into two streams, first feed stream 4 and second feed stream 7 defining a second feed stream enthalpy. First feed stream 4, which is the larger of the two streams and contains about 82% of LNG stream 3, is heated in first exchanger 5 to about −129° F. to produce first tower feed stream 6 defining a first tower feed stream enthalpy. First tower feeds stream, which is still substantially all-liquid, and second feed stream 7 are sent to fractionation tower 10. In a preferred embodiment, second feed stream 7 is introduced to fractionation tower 10 at a position above the first tower feed stream. In one embodiment, second feed stream 7 is introduced as a top feed to the fractionation tower 10. In one embodiment, first tower feed stream 6 is introduced as a bottom feed to the fractionation tower.

In FIG. 5, tower overhead stream 14 is compressed in lean gas compressor 27 to about 534 psia. The partially boosted tower overhead stream 28 is then cooled to about −131.1° F. and is completely condensed in first exchanger 5 to produce low-pressure lean LNG stream 21. Low pressure lean LNG stream 21 is then pumped preferably by pump 22 to elevate its pressure to about 1422 psia. Stream 23 leaving pump 22 is the high pressure lean LNG product stream that can be sent for vaporization and/or energy recovery.

As another embodiment of the present invention, FIG. 6 shows a dual lean reflux scheme that utilizes a side reboiler 31 to increase NGL recovery from LNG stream 1. In addition to the elements shown in FIG. 5, side reboiler 31 introduced in FIG. 6 advantageously maximizes utilization of cold streams and also minimizes compressor power requirements. As in other embodiments described herein, rich low pressure LNG stream 1 is pumped by LNG pump 2 to about 485 psia. LNG stream 3 is then split into two streams, first feed stream 4 and second feed stream 7. First feed stream 4, which is the larger of the two streams and contains about 92% of LNG stream 3, is heated in first exchanger or heater 5 to about −128.4° F. to produce first tower feed stream 6. First tower feed stream, which is still substantially all liquid, is sent to fractionation tower 10. IN one embodiment first tower feed stream is fed to the fractionation tower as a bottom feed stream. Second feed stream 7 is introduced preferably as a middle feed stream to fractionation tower 10. Tower overhead stream 14 is compressed in lean gas compressor 27 to about 543 psia. The partially boosted tower overhead stream 28 is then cooled in side reboiler 31 to about −124.1° F.

To provide a portion of the reboiling energy for fractionation tower 10, side reboiler 31 is utilized in this embodiment. Cold tower liquid stream or tower side reboiler stream 29 is heated in side reboiler 31 and returned to fractionation tower 10 as stream 30. Partially cooled stream 32 is then further cooled to about −130.7° F. and completely condensed in first exchanger 5 to produce low pressure lean LNG stream 21. Although two separate exchangers are shown to provide cooling for tower overhead stream 28, a single exchanger can be used. Low pressure lean LNG stream 21 is then pumped, preferably by pump 22, to elevate its pressure to about 1422 psia to produce high pressure lean LNG stream 23. Lean LNG stream 23 is then split into a lean tower reflux stream 25 and lean LNG product stream 48. Lean LNG product stream 48 is sent for vaporization and/or energy recovery. Lean tower reflux stream 25 is then cooled in first exchanger 5 to about −232° F. and is then sent to fractionation tower 10, preferably as top feed stream 26.

In this embodiment, the compressed tower overhead stream 28 can cooled by heat exchange with at least a portion of the first feed stream 4 thereby providing at least a portion of the duty for the step of preheating at least a portion of the first feed stream 4.

FIG. 6 demonstrates an embodiment including splitting the high pressure lean LNG stream 23 to create a lean tower reflux stream 25 and a lean LNG product stream 48. The lean tower reflux stream 25 is then cooled to produce a cooled lean tower reflux stream 26, which is fed to the fractionation tower 10.

The embodiments of the invention described herein are applicable when the heavy hydrocarbon components include C2 components, C3 components, and heavier components, i.e., ethane recovery. The embodiments described herein are applicable also when the heavy hydrocarbon components include C3 components, and heavier components, i.e., propane recovery.

Higher recoveries are made possible by increasing amount of flow in first tower feed stream 6, which is cold and rich in the embodiments shown in FIG. 5 and FIG. 6. Recoveries are also enhanced by increase in amount of cooled lean tower reflux stream 26 that is returned to fractionation tower 10. Increasing the discharge pressure of lean gas compressor 27 eliminates exchanger pinch in first exchanger 5 and avoids two phase LNG stream from reaching pump 22. The tower pressure can also be lowered to increase recovery, at the cost of higher compression power. This scheme is able to give high ethane recovery with very high propane recovery. Comparing results through process simulation with prior art, it can be seen that for a modest increase in power, the process in FIG. 6 recovers more ethane. In addition, the recovery of propane is also increased. Although the simulations have been carried out for C2+ (ethane, ethylene, propane, propylene and heavier hydrocarbons) component recovery, the same process can be used for C3+ (propane, propylene and heavier hydrocarbons) component recovery.

As yet another embodiment of the present invention, FIG. 7 shows a scheme that uses first cooler 8 as an overhead condenser along with a lean gas compressor 27 to recover NGL from rich low pressure LNG 1. In this embodiment, rich low pressure LNG 1 is pumped by LNG pump 2 to about 525 psia. LNG stream 3 is then split into two streams, first feed stream 4 and second feed stream 7. First feed stream 4, which is the larger of the two streams and contains about 61% of LNG stream 3, is heated in first exchanger 5 to about −126.8° F. to produce first tower feed stream 6. First tower feed stream 6, which is substantially all liquid, is introduced to fractionation tower 10, preferably as a bottom feed stream. Second feed stream 7 is heated in first cooler 8 to about −10° F. to produce second tower feed stream 9. Second tower feed stream 9 is sent, preferably at a position lower than first tower feed stream 7, to fractionation tower 10. In one embodiment, second tower feed stream 9 is a bottom feed.

Tower overhead stream 14 is cooled to about −133.3° F. in first cooler 8 thereby partially condensing tower overhead stream. Partially condensed tower overhead stream 15 is sent to reflux accumulator or separator 16. Partially condensed tower overhead stream 15 is then separated into lean vapor stream 17 and tower reflux stream 18. Tower reflux stream 18 is pumped by pump 19 and is sent to fractionation tower 10, preferably as a top feed stream. Lean vapor or residue stream 17 is compressed in lean gas compressor 27 to about 596 psia to produce partially boosted compressed overhead stream 28. Partially boosted compressed overhead stream 28 is cooled, preferably in side reboiler 31 (see FIG. 6), to about −121.5° F. Alternately, as shown in FIG. 7, partially boosted compressed overhead stream 28 can be cooled in first cooler 8 to produce partially cooled stream 32. Cold tower side reboiler stream 29 exchanges heat in first cooler 8 and returned to fractionation tower 10 as return stream 30. Partially cooled stream 32 is then further cooled to about −125.3° F. and completely condensed in first exchanger 5 to produce low pressure lean LNG stream 21. Lean LNG stream 21 is then pumped preferably by pump 22 to elevate its pressure to about 1422 psia to produce lean LNG product stream 23. Stream 23 is the high pressure lean LNG product that is sent for vaporization and/or energy recovery.

Tower reflux stream 18 can be subcooled to enhance recovery. This subcooling can take place in first exchanger 5.

FIG. 8 illustrates another embodiment for recovery of NGL products from a rich lower pressure LNG stream. In this embodiment, overhead expander 33 is used to generate reflux for fractionation tower 10 to recover NGL. Rich low pressure LNG stream 1 is pumped in LNG pump 2 to about 550 psia. LNG stream 3 exiting pump 2 is heated in first exchanger 5 to about −125.5° F. LNG stream 3, which is still all liquid, is introduced to fractionation tower 10. In one preferred embodiment, LNG stream 3 is introduced to fractionation tower 10 as a bottom feed stream.

At least a portion of tower overhead stream 14 is expanded in expander 33 to about 365 psia so that tower overhead stream 14 is at least partially condensed thereby producing partially condensed low pressure vapor stream 35. Partially condensed low pressure vapor stream 35 is then sent to reflux accumulator or separator 16 where the stream is separated into lean vapor stream 17 and tower reflux stream 18. Tower reflux stream 18 is pumped by pump 19. Tower reflux stream 18 is then subcooled in first exchanger 5 to about −232° F. to produce lean tower reflux stream 26. Lean tower reflux stream 26 is then sent to fractionation tower 10, preferably as a top feed stream. Lean vapor stream 17 is boosted in pressure booster compressor 34, which is driven off or powered by the power of expander 33, and then further compressed in lean compressor 27 to about 526 psia. Compressed and warm residue gas stream 28 is cooled in side reboiler 31 to about −115.8° F. to produce partially cooled stream 32. Cold tower side reboiler stream 29 is heated in side reboiler 31 and returned to fractionation tower 10 as return stream 30. Partially cooled stream 32 is then further cooled to about −132.6° F. and completely condensed in first exchanger 5 to produce lean LNG stream 21. Lean LNG stream 21 is pumped by pump 22 to elevate its pressure to about 1422 psia. Lean LNG product stream 23 is the high pressure lean LNG product that is sent for vaporization and/or energy recovery.

FIG. 9 illustrates yet another embodiment that is used to separate rich, low pressure LNG stream 1 by utilizing a subcooled lean LNG stream to increase NGL recovery and lean gas compressor 27 for a portion of a second lean vapor stream 17. In this embodiment, rich low pressure LNG stream 1 is pumped, preferably by pump 2, to about 535 psia. LNG stream 3 exiting pump 2 is then heated in first exchanger 5 to about −133.4° F. to produce first tower feed stream 6. First tower feed stream 6, which is still all liquid, is introduced to fractionation tower 10, preferably as a bottom tower feed stream.

Tower overhead stream 14 is cooled to about −133° F. and partially condensed in first exchanger 5 to produce two phase stream or partially condensed tower overhead stream 15. Two phase stream 15 is sent to suction scrubber or second separator 38 that separates two phase stream 15 into second lean vapor stream 17 and lean LNG stream 21, which is liquid.

Lean LNG stream 21 is elevated in pressure, preferably by pump 22, to 1422 psia to produce a high pressure lean LNG stream 23. High pressure lean LNG stream 23 is then split into lean tower reflux stream 25 and lean LNG product stream 48. Lean tower reflux stream 25 is cooled in first exchanger 5 to about −232° F. and is then sent, preferably as lean tower reflux stream 26 as a top feed to fractionation tower 10.

Second lean vapor stream 17 is compressed in lean gas compressor 27 to produce a compressed second lean vapor stream 37. Compressed second lean vapor stream 37 is then combined with lean LNG product stream 48 to produce the lean LNG product that is sent for vaporization and/or energy recovery. If high pressure lean LNG stream 23 is sent for energy recovery, then second lean vapor stream 17 can be cooled in side reboilers that can be added to fractionation tower 10, similar to that shown in FIG. 6. With the additional cooling for second lean vapor stream 17, energy recovery will be more efficient due to the stream being colder.

COMPARISON OF PRIOR ART PROCESSES AND EMBODIMENTS OF THE PRESENT INVENTION

	TABLE II
	POWER		Tower	LP
	C2	C3	NGL	Pump	Compressor	Total	UA	Power	LNG
FIG.	RECOVERY %	RECOVERY %	BPD	hp	hp	hp	Btu/hr-F	Psia	° F.	Hp/gpm
1	87.63	98.48	41271	8047		8047	4.44E+06	510	−134.5	6.69
2	88.87	97.73	41520	8055		8055	4.25E+06	510	−133.7	6.65
3	88.57	99.32	41635	8304		8304	4.54E+06	510	−133.7	6.84
4	88.05	97.68	41282	7528	2195	9723	5.43E+06	435	−137	8.08
5	92.48	98.41	42658	7789	2195	9984	5.48E+06	460	−131.1	8.02
6	94.74	99.62	43464	8279	2195	10474	5.93E+06	470	−130.7	8.26
7	98	99.99	44457	8257	2195	10452	5.30E+06	510	−125.3	8.06
8	96.83	100	44116	8161	2195	10356	6.98E+06	535	−132.6	8.05
9	96.03	99.99	43882	8097	2186	10283	4.73E+06	520	−133	8.03

Table II provides a side-by-side comparison of prior art processes and described embodiment of the present invention. As can be seen in Table II, the prior art process shown in FIG. 4 has the lowest recovery rates for both C2 and C3 recoveries when compared to the process embodiments described herein.

The invention also encompasses the apparatus necessary for each process embodiment. A preferred embodiment of the apparatus for separating a liquefied natural gas (LNG) stream into a more volatile fraction containing a substantial amount of the methane and lighter components and a less volatile fraction containing a large portion of the heavy hydrocarbon components includes means for splitting LNG stream 3 into a first feed stream 4 and a second feed stream 7. Various means are known in the art from a simple T in a line to more complex vessels. First exchanger 5 is operable to preheat first feed stream 4 thereby creating first tower feed stream 6. First tower feed stream is fed into fractionation tower 10. Fractionation tower 10 produces tower overhead stream 14 including the more volatile fraction containing the substantial amount of the methane and lighter components and a tower bottoms stream 12 including the less volatile fraction containing the heavy hydrocarbon components. At least a portion of tower overhead stream 14 is cooled by first cooler 8, which is operable to cool and partially condense at least a portion of the tower overhead stream 14 to produce a partially condensed tower overhead stream 15. In a preferred embodiment, first cooler 8 is operable to allow for heat exchange between the portion of the tower overhead stream 14 and the second feed stream 7. Separator 16 is provided which is operable to separate the partially condensed overhead tower stream 15 into a lean vapor stream 17 and a tower reflux stream 18. Tower reflux stream 18 returns to and is fed to the fractionation tower 10. In a preferred embodiment, first exchanger 5 is operable to allow for heat exchange between the lean vapor stream 17 and the first feed stream 4, the lean vapor stream 17 being cooled in the first exchanger 5 to provide a lean LNG stream 21. Pump 22 is operable for pumping the lean LNG stream 21 to a higher pressure to produce high pressure LNG stream 23.

In another embodiment, first compressor 27 is operable to receive lean vapor stream 17 and boost the pressure to produce compressed overhead stream 28. Another embodiment includes side reboiler for supplying at least a portion of reboiling requirements for the fractionation tower.

While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.

Claims

1. A process for separating a liquefied natural gas (LNG) stream containing methane and lighter components and heavy hydrocarbon components into a more volatile fraction containing a substantial amount of the methane and lighter components and a less volatile fraction containing a large portion of the heavy hydrocarbon components, the process comprising the steps of: (a) splitting the LNG stream into a first feed stream and a second feed stream defining a second feed stream enthalpy; (b) preheating at least a portion of the first feed stream to provide first tower feed stream defining a first tower feed stream enthalpy; (c) supplying the first tower feed stream to a fractionation tower to produce a tower overhead stream including the more volatile fraction containing the substantial amount of the methane and lighter components and a tower bottoms stream including the less volatile fraction containing the heavy hydrocarbon components; (d) supplying second feed stream to the fractionation tower such that the first feed stream and the second tower feed stream have a substantially common composition and the first tower feed stream enthalpy differs from the second feed stream enthalpy; (e) compressing the tower overhead stream to produce a compressed overhead stream; (f) cooling the compressed overhead stream such that the compressed overhead stream is substantially condensed thereby producing a lean LNG stream; and (g) pumping the lean LNG stream to a high pressure lean LNG stream.

2. The process according to claim 1, wherein the heavy hydrocarbon components include C2 components, C3 components and heavier components.

3. The process according to claim 1, wherein the heavy hydrocarbon components include C3 components and heavier components.

4. The process according to claim 1, wherein the step of supplying the fractionation tower with the second feed stream includes supplying the fractionation tower with the second feed stream at a position in the fractionation tower higher than the position where the first tower feed stream enters the fractionation tower.

5. The process according to claim 1, wherein the step of cooling the compressed tower overhead stream includes cooling the compressed tower overhead stream by heat exchange with at least a portion of the first feed stream thereby providing at least a portion of the duty for the step of preheating at least a portion of the first feed stream.

6. The process according to claim 1, further comprising the steps of splitting the high pressure lean LNG stream to create a lean tower reflux stream and a lean LNG product stream, cooling the lean tower reflux stream to produce a cooled lean tower reflux stream, and feeding the cooled lean tower reflux stream to the fractionation tower.

7. A process for separating a liquefied natural gas (LNG) stream containing methane and lighter components and heavy hydrocarbon components into a more volatile fraction containing a substantial amount of the methane and lighter components and a less volatile fraction containing a large portion of the heavy hydrocarbon components, the process comprising the steps of: (a) splitting the LNG stream into a first feed stream and a second feed stream and supplying the second feed stream to a fractionation tower as a second tower feed stream to produce a tower overhead stream including the more volatile fraction containing the substantial amount of the methane and lighter components and a tower bottoms stream including the less volatile fraction containing the heavy hydrocarbon components; (b) preheating at least a portion of the first feed stream to produce a first tower feed stream and supplying the first tower feed stream to the fractionation tower; (c) compressing the tower overhead stream to produce compressed overhead stream; (d) cooling the compressed overhead stream so that the compressed overhead stream is substantially condensed thereby producing a lean LNG stream; (e) pumping the lean LNG stream to a high pressure to produce a high pressure lean LNG stream; (f) splitting the high pressure lean LNG stream into a lean tower reflux stream and a lean LNG product stream; (g) cooling the lean tower reflux stream to produce a cooled lean tower reflux stream; and (h) supplying the cooled lean tower reflux stream to the fractionation tower.

8. The process according to claim 7, wherein the heavy hydrocarbon components include C2 components, C3 components and heavier components.

9. The process according to claim 7, wherein the heavy hydrocarbon components include C3 components and heavier components.

10. The process according to claim 7, wherein the step of supplying the fractionation tower with cooled lean tower reflux stream includes supplying the cooled lean tower reflux stream to a top portion of the fractionation tower.

11. The process according to claim 7, wherein the step of supplying the second tower feed stream to the fractionation tower includes supplying the second tower feed stream to the fractionation tower as a tower middle feed stream.

12. The process according to claim 7, wherein the step of supplying the cooled lean tower reflux stream to the fractionation tower includes supplying the cooled lean tower reflux stream to a top of the fractionation tower.

13. The process according to claim 7, wherein the step of cooling the tower overhead stream includes cooling the tower overhead stream by heat exchange contact with at least a portion of the first feed stream thereby providing at least a portion of the duty for the step of preheating at least a portion of the first feed stream.

14. The process according to claim 7, wherein the step of cooling the lean tower reflux stream includes cooling the lean tower reflux stream by heat exchange contact with the first feed stream thereby providing at least a portion of a duty for the step of preheating at least a portion of the first feed stream.

15. The process according to claim 7, wherein the step of cooling the tower overhead stream includes cooling the tower overhead stream by heat exchange contact with a tower side reboiler stream thereby providing at least a portion of a duty operable to provide reboiling for the fractionation tower.