US20120067079A1 - Nitrogen rejection and liquifier system for liquified natural gas production - Google Patents

Abstract

A method for recovering liquefied natural gas from a gas mixture containing natural gas and impurities by subjecting the natural gas to a series of steps beginning with feeding a natural gas stream containing impurities to a nitrogen rejection unit; feeding the purified natural gas stream to a liquefier heat exchanger; expanding the liquefied natural gas and feeding the expanded liquefied natural gas to a flash vessel; flashing the liquid natural gas and separating the liquefied natural gas from the flash gas; and feeding the liquefied natural gas to storage and the flash gas to the nitrogen rejection unit.

Description

CROSS REFERENCE TO RELATED APPLICATIONS
• This application claims priority from U.S. provisional patent application Ser. No. 61/317,466, filed Mar. 25, 2010.
BACKGROUND OF THE INVENTION
• The invention relates to the integration of a liquefied natural gas (LNG) liquefier system with a nitrogen rejection unit (NRU) so as to minimize the capital and operating costs while maintaining liquefied natural gas product purity requirements.
• Renewable methane can be recovered from a number of sources, such as anaerobic digestion of municipal or industrial waste streams, the degradation of biomass in landfills, the gasification of waste and biomass streams, amongst others. In many instances, this renewable methane require purification before it can be used and/or sold into higher valued markets, such as injection into the pipeline grid, as a feedstock for liquefied natural gas, as a vehicle fuel, or as a feedstock for the production of hydrogen. Further, the energy that is required to purify the renewable methane is significant.
• The cleanup of biogas/landfill gas is both capital and power intensive because it contains a large number of trace and bulk contaminants in fairly large concentrations. Various methods are employed to remove these including chilling, cryogenic methods and various adsorption and scrubbing processes. However, these processes can be expensive in both capital and operating costs and it is important to minimize these costs to achieve an economically viable process.
• A typical process for the purification of the methane from biogas/landfill gas requires several steps. Sulfur removal is generally followed by drying. The dried gas stream is then treated for contaminants such a volatile organic compounds by process such as adsorption, CO₂washing or by cryogenic methods. The stream is then treated for bulk carbon dioxide removal by a membrane or adsorption process and then is treated for removal of nitrogen. All these purification steps are necessary before the biogas/landfill gas can be liquefied and stored in anticipation of being dispensed, or directed towards other uses, such as pipeline injection, energy production with fuel cells or small-scale hydrogen production. LNG production is particularly challenging since all condensable contaminants including carbon dioxide must be removed to low ppm levels.
• The invention will allow for maximizing the methane recovery while maintaining high liquefied natural gas product purity. The operator can utilize a smaller nitrogen rejection unit and can optimize power consumption. The process of using the nitrogen rejection unit integrated with the liquefied natural gas liquefier system achieves greater product purity (>96 mol % methane) and greater than 89% methane recovery than conventional non-integrated combinations.
• By integration of the liquefied natural gas liquefier system with a nitrogen rejection unit, the overall system becomes more compact and efficient. This further enables the operator to maximize methane recovery while maintaining high liquefied natural gas product purity while enable a smaller nitrogen rejection unit. The invention further allows the operator to optimize power consumption while allowing for significantly higher product purity and methane recovery than conventional or unitegrated NRU and liquefier combinations which are limited to 96 mol % methane and 80% methane recovery.
SUMMARY OF THE INVENTION
• The invention is a method for recovering liquefied natural gas comprising the steps:
• Feeding a natural gas stream containing impurities to a nitrogen rejection unit;
  Feeding the purified natural gas stream to a liquefier heat exchanger;
  Expanding the liquefied natural gas and feeding the expanded liquefied natural gas to a flash vessel;
  Flashing the liquid natural gas and separating the liquefied natural gas from the flash gas;
  Feeding the liquefied natural gas to storage and the flash gas to said nitrogen rejection unit.
• Alternatively, the invention is a method for recovering liquefied natural gas comprising the steps:
• Feeding a natural gas stream containing impurities to a nitrogen rejection unit;
  Feeding the purified natural gas stream to a liquefier heat exchanger;
  Expanding the liquefied natural gas and feeding the expanded liquefied natural gas to a flash vessel;
  Flashing the liquid natural gas and separating the liquefied natural gas from the flash gas;
  Recovering refrigeration from said flash gas; and
  Feeding the liquefied natural gas to storage and the flash gas to said nitrogen rejection unit.
• The invention further comprises an apparatus comprising a nitrogen rejection unit, a liquefier heat exchanger and a flash vessel.
• The raw feed gas is first compressed and pre-conditioned which entails the removal of water, carbon dioxide, non-methane organic compounds (NMOCs) and sulfur compounds by known methods. The partially purified gas is fed to the nitrogen rejection unit where much of the nitrogen is rejected. Since product purity and methane recovery are inversely related, nitrogen rejection is limited to maximize the methane recovery for the smallest equipment cost. The resulting gas which contains significantly lower amounts of inerts is fed to the liquefier heat exchanger where it is liquefied at pressure to a subcooled state. Typical pressures range from 30 bar to 6 bar with a tradeoff between mixed refrigeration compressor power and compression power for the purification system. This liquid is expanded through a valve whereby further cooling is effected to about 2 bar (range is 1 to 5 bar).
• The two-phase mixture is separated in a flash vessel and the resulting liquid is directed to the storage tanks, while the flash gas which is richer in nitrogen is recycled back to the nitrogen rejection unit. The flash gas can also be combined with the raw natural gas/biogas at the front end of the overall process if the nitrogen rejection unit does not have a recycle compressor. Clearly additional flash gas from the storage tank will be produced. This too is recycled back to the nitrogen rejection unit or to the front end of the cleanup process. The only methane that will be lost is the nitrogen rejection unit waste stream which is nitrogen-rich but otherwise very pure and can be flared or converted into power using a gas engine or a fuel cell.
• The end flash from the flash vessel has an additional advantage in that the liquid outlet of a flash is at equilibrium which implies that it will produce some more gas inside the line between the end flash and storage tank because of product line pressure drop. Therefore, it is best practice that the flash pressure be lower than the storage pressure. To maximize liquefied natural gas production and minimize product flash losses, the first flash within the flash vessel is effected at a pressure lower than the storage tank pressure, whence, the liquid coming down from the end-flash tank will be sub-cooled at storage pressure. A cryogenic pump can be utilized to overcome this concern, but involves additional cost, maintenance and potential reliability issues. Therefore it is advisable to have horizontal storage tanks, and a cold box layout so that the liquid level inside the end flash will be higher by a few hundreds of mbar of equivalent liquefied natural gas head that the top of the storage and to use that additional head to pressurizeFIG. 2c.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a typical mixed refrigerant liquefied natural gas liquefier.
• FIG. 2ais an integrated mixed refrigerant liquefied natural gas liquefier.
• FIG. 2bdepicts a different embodiment of an integrated mixed refrigerant liquefied natural gas liquefier.
• FIG. 2cdepicts another embodiment of an integrated mixed refrigerant liquefied natural gas liquefier.
• FIG. 3 shows methane recovery versus nitrogen rejection unit product methane content.
• FIG. 4 shows a liquefier embodied in the invention.
• FIG. 5ashows a liquefier having a lower cost to operate.
• FIG. 5bshows a different embodiment of a lower cost to operate liquefier.
DETAILED DESCRIPTION OF THE INVENTION
• Landfill gas is purified and all the water, sulfur compounds, NMOCs and carbon dioxide are removed in a pre-purification process. The purified gas contains methane, nitrogen and oxygen and has the following composition:

• TABLE 1
  NRU Feed Gas Composition

  	Species	Mole Fraction
  	Carbon Dioxide	0.0100
  	Nitrogen	0.2160
  	Methane	0.7720
  	Oxygen	0.0020

• The gas is further purified in an adsorption system so that the carbon dioxide level is reduced below 50 ppmv and a large portion of the nitrogen is removed. Oxygen usually does not adsorb appreciably and about 50% of the oxygen is removed in each case.FIG. 1 illustrates a typical MR liquefier stream without process integration. In this case both storage tank losses and nitrogen rejection unit waste streams are not recovered.
• Turning to the figures,FIG. 1, represents a base case mixed refrigerant liquefied natural gas liquefier. Purified natural gas is fed throughline1 through main heat exchanger A where it will be warmed and fed through valve V1 andline2 as liquefied natural gas to a storage container, not shown.
• A cold water stream (CWS) is fed throughline11 to heat exchanger E as well as through valve V6 andline13 toline12 as cold water return (CWR). The cold mixed refrigerant is fed throughline9 to knockout drum B where it will proceed throughline7A to refrigerant pump C and through open valve V4 to contactline3 in heat exchanger A. When valve V4 is closed and valve V5 is open the mixed refrigerant will re-enter knock out drum B throughline7. The overhead from knockout drum B will travel throughline3 to heat exchanger A and enter valve V2 to column unit D where it will exit unit D through overhead line4 as well as through the bottom of unit D throughline5A. This will exit heat exchanger A throughline5 and connect to inlet separator G where the bottoms will travel through line8 and transfer pump H toline6 which will enter the knockout drum B. The refrigerant will leave the inlet separator G through line8A and connect to mixed refrigerant column unit F where the mixed refrigerant will travel throughline10 back to heat exchanger E.
• InFIG. 2a, an integrated mixed refrigerant liquefied natural gas liquefier system is shown per the operation of the invention. Nitrogen containing biogas or other source of natural gas such as landfill gas feed is fed throughline23 to nitrogen rejection unit R which is typically a vacuum swing adsorption (VSA) system. Waste gas is fed throughline25 to blower S and released into the atmosphere. Depressurization gas is released throughline25A intoline22 where it will travel through recycle compressor Q and reenter the nitrogen containing landfill orbiogas feed line23.
• The nitrogen recovery unit product/liquefier feed natural gas is fed throughline24 into heat exchanger I where it will pass through valve V12 and enter flash tank J. The now liquefied natural gas will exit through valve V11 andline25 toline26 where it will enter storage tank K and can be accessed throughline21 and valve V10 for later use. Vent gas from the storage tank K will exit through line20 where it will joinline22 and be fed back through the recycle compressor Q to the nitrogen containingbiogas feed line23.
• Heat exchanger T is fed cold water throughline35A to provide a cooling medium which will also feed to the coldwater return line35 through valve V16.Line24A directs warm water leaving heat exchanger T. The cold refrigerant is fed throughline36 to knock out drum P which feeds the cold refrigerant to the heat exchanger I throughline28 and which passes through valve V13 to the column unit L where refrigerant from the top exits throughline31 and through the bottom through31A which joinsline31 and passes through heat exchanger I andline31 will be fed to inlet separator M where refrigerant exits throughline33 and is fed to mixed refrigerant column unit U which feeds mixed refrigerant, now warmer to the heat exchanger T throughline24. The bottoms from the inlet separator M is fed throughline32 and transfer pump N back to knock out drum P. The bottoms from the knockout drum P are fed throughline30 and refrigerant pump O to valve V15 for reentry back into the knockout drum P. BZ designates the cold box boundary.
• The refrigerant from the knockout drum P may also enterline29 and open valve V14 where it will feed intoline31 and entry into the inlet separator M.
• FIG. 2bis a similar version of the integrated mixed refrigerant liquefied natural gas liquefier system ofFIG. 2A with the numbering being the same asFIG. 2A. This embodiment has compressor Q online23 rather thanline22 and noreturn line25A from the nitrogen rejection unit R toline22. Also,line30A connects with valve V15A to heat exchanger I such that refrigerant from knock out drum P is directed to the heat exchanger I.
• FIG. 2cis another embodiment of the invention showing an integrated mixed refrigerant liquefied natural gas liquefier. Natural gas such as that from landfill gas or biogas from a nitrogen recovery unit, not shown, is fed throughline40 into heat exchanger V. The natural gas is liquefied and its pressure is higher as it exits throughline40A through temperature control valve V17. The liquefied natural gas enters end flash unit W where the flashed liquefied natural gas is fed throughline41 and open pressure control valve V18 and recycled back to heat exchanger V where it will exit and be fed throughline42 to a nitrogen recovery unit, not shown.
• The bottoms from the flash unit W exit throughline43 and open valve V19 where it will enter horizontal cryogenic storage tank Y. Additional static head is maintained between the liquid level in the end flash unit W and the horizontal cryogenic storage tank Y such that it is equivalent to subcooling at storage level and pressure. Line BZ represents the cold box boundary.
• FIG. 3 shows the effect of methane product purity on methane recovery. Methane recovery decreases as the nitrogen recovery unit product methane content in mole % increases.
• FIG. 4 shows a preferred liquefier embodiment. Natural gas such as that found in landfill gas or biogas is fed throughline64 to heat exchanger AA where it will exit as liquefied natural gas through open valve V20 and be fed to flash tank AB. The liquefied natural gas from the bottoms of the flash tank AB will exit throughline66 and open valve V22 where it will be fed to storage, not shown. The gaseous natural gas tops of the flash tank will exit throughline65 and re-enter heat exchanger AA where it will be fed to a mixed gas nitrogen recovery unit, not shown.
• Cold water is fed throughline60 into heat exchanger AI to provide a cooling medium and also fed throughline61 and open valve V25 to the coldwater return line62. Refrigerant will exit throughline64 and be fed through to a knockout drum AD where refrigerant is fed throughline51 and refrigerant pump AE through open valve V24 to line52 passing through heat exchanger AA. When valve V24 is closed and valve V24A is open, the refrigerant is fed throughline55 back to knockout drum AD. Refrigerant is also fed throughline56 from the top of the knockout drum AD toline52 passing through heat exchanger AA.Line52 will deliver the refrigerant through open valve V21 to a column unit AC where the bottoms from said unit are fed throughline53 to rejoin with the tops which exit unit AC throughline54.Line54 passes through heat exchanger AA where it will be fed to inlet separator AF.
• The refrigerant inline54 is occasionally supplemented from the knockout drum AD through open valve V23 andline57 which connects with the tops from the knockout drum AD throughline56.Line54 will enter the inlet separator AF where its bottoms are transferred throughline58A and transfer pump AG toline50 which returns to the knockout drum AD. The tops from the inlet separator AF exit throughline58 and enter mixed refrigerant column unit AH where mixed refrigerant will enter the heat exchanger AI for cooling and reentry into the knockout drum AD for entry into heat exchanger AA.
• FIG. 5ashows a lower cost embodiment liquefier. Natural gas such as that found in landfill gas or biogas is fed throughline79 and open valve V30 where it will enter flash tank BA. Liquefied natural gas exits throughline77 and open valve V33 to storage, not shown. Natural gas will exit the flash tank BA throughline78 where it will pass through economizer BC and exit to a nitrogen recovery unit, not shown. Valve V32 can be opened and excess nitrogen can be recovered through line78A, unit TIC back into flash tank BA.
• Part of the natural gas feed fromline78 is fed through open valve V31 to line76 which passes through heat exchanger BD and open valve V34 back to the flash tank BA as liquefied natural gas.
• Cold water is fed throughline83 to heat exchanger BJ and throughline85 and open valve V39 to coldwater return line84. Refrigerant will exit throughline85A and be fed to knockout drum BH where it will exit through the bottom of the knockout drum through open valve V36 and refrigerant pump BI to be fed to line74 passing through heat exchanger BD. Valve V36 can be closed and valve V38 open such that refrigerant will pass throughline75 back to knockout drum BR
• The tops from the knockout drum BH will be fed throughline70 to line74 passing through heat exchanger BD. The refrigerant will pass through open valve V35 and be fed to column unit BE where the bottoms from the unit exit throughline71 and join with the tops from the unit BEline72 which passes refrigerant through heat exchanger BD. This refrigerant will enter inlet separator BG throughline72 where the bottoms from the inlet separator BG are fed throughline80 and transfer pump BF back to the knockout drum BH.
• The tops from the inlet separator will exit throughline81 to mixed refrigerant unit BK. The mixed refrigerant from unit BK is fed back to heat exchanger BJ as a warm fluid throughline82 where it will be cooled down and ultimately fed back into heat exchanger BD after passing through knockout drum BH. Line BZ designates the cold box boundary.
• FIG. 5bis virtually identical toFIG. 5adesignating a lower cost liquefier embodiment. In this embodiment, the numbering is the same and there is no return embodiment on top of the flash tank BA, thus line78A, valve V32 and TIC control mechanism are not present. InFIG. 5b, the cold box boundary BZ is also broader and covers the flash tank BA which is not seen inFIG. 5a.
• Typical nitrogen rejection performance is shown inFIG. 3 for a vacuum swing adsorption (VSA) nitrogen rejection unit. The invention is shown inFIG. 2a. The nitrogen rejection amount was varied while ensuring that the final LNG product contained 98%+methane. Three cases were considered for illustrative purposes where the NRU product/liquefier feed gas contained 90.6, 98.2 and 98% methane (C1). The relative equipment size, which determines capital cost and the power were calculated and compared. The results are as indicated in Tables 2 and 3 below. In Table 2, both the pre-cleanup system, which is used to remove all contaminants other than nitrogen and oxygen, and the four bed VSA system are compared in terms of size which is directly proportional to the kg-moles/hr of NRU feed to be processed or the nitrogen to be rejected.Case 3 clearly shows significant benefits when a less pure NRU product is fed to the liquefier with a pre-cleanup system that is 17% smaller and a NRU that is 23% smaller than the first case.

• TABLE 2
  Effect of Liquefier Feed Composition on Overall Methane
  Recovery and Equipment Size

  	C1 in Liquefier	Pre-Cleanup	NRU	Wobbe Index
  	Feed (mol %)	Relative Size	Relative Size	(MJ/m3)

  Case 1	98.0	1.17	1.23	50.37
  Case 2	96.2	1.06	1.10	50.11
  Case 3	90.6	1.00	1.00	49.35

• In addition, the relative power for all 3 cases is compared in Table 3 which shows that the extra power needed for liquefaction and recycle with higher inerts (case 1) is compensated by the vacuum pump power needed for higher NRU purity (case 3). Hence, there is no appreciable net power penalty.

• TABLE 3
  Effect of Liquefier Feed Composition on Net Power

  	C1 in Liquefier Feed	Relative Power
  	(mol %)	(%)

  	Case 1	98.0	100.5
  	Case 2	96.2	99.6
  	Case 3	90.6	100.0

• Other embodiments of the invention are illustrated inFIGS. 5aand5b, both of which are lower capital cost options and do not require a separate pass in the main heat exchanger, or a larger coldbox. Nevertheless, both embodiments do not allow for full cold recovery and are less efficient. Additionally, if all the purified natural gas from the NRU is fed to the economizer, a very large temperature gradient will result at the cold end of this exchanger. Therefore, it is desired that only a portion of the NRU product is fed to the economizer so that it can be liquefied, or cooled close to the liquefaction temperature. The portion of the NRU product gas cooled in the economizer can be sent to flash tank labeled BA inFIGS. 5aand5bas sub-cooled liquid or to the main heat exchanger.
• While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.

Claims (32)

Having thus described the invention, what we claim is:

1. A method for recovering liquefied natural gas comprising the steps:

feeding a natural gas stream containing impurities to a nitrogen rejection unit;

feeding the purified natural gas stream to a liquefier heat exchanger;

expanding the liquefied natural gas and feeding the expanded liquefied natural gas to a flash vessel;

flashing the liquid natural gas and separating the liquefied natural gas from the flash gas; and

feeding the liquefied natural gas to storage and the flash gas to said nitrogen rejection unit.

2. The method as claimed inclaim 1 wherein said impurities are selected from the group consisting of water, carbon dioxide, non-methane organic compounds and sulfur compounds.

3. The method as claimed inclaim 1 wherein nitrogen is recovered from said nitrogen rejection unit.

4. The method as claimed inclaim 1 wherein the pressure in said liquefier heat exchanger range from 6 to 30 bar.

5. The method as claimed inclaim 1 wherein said liquefied natural gas is expanded to a pressure of 1 to 5 bar.

6. The method as claimed inclaim 1 wherein said flash gas is richer in nitrogen than natural gas.

7. The method as claimed inclaim 1 wherein said natural gas stream is selected from the group consisting of landfill gas and biogas.

8. The method as claimed inclaim 1 wherein said flash gas is recycled to said natural gas feed.

9. The method as claimed inclaim 1 wherein flash pressure is lower than storage pressure.

10. The method as claimed inclaim 1 wherein said nitrogen rejection unit is a vacuum swing adsorption unit.

11. The method as claimed inclaim 1 wherein said recovered natural gas is fed to a storage unit.

12. The method as claimed inclaim 1 wherein said storage unit is situated horizontally.

13. A method for recovering liquefied natural gas comprising the steps:

feeding a natural gas stream containing impurities to a nitrogen rejection unit;

feeding the purified natural gas stream to a liquefier heat exchanger;

expanding the liquefied natural gas and feeding the expanded liquefied natural gas to a flash vessel;

flashing the liquid natural gas and separating the liquefied natural gas from the flash gas;

recovering refrigeration from said flash gas; and

feeding the liquefied natural gas to storage and the flash gas to said nitrogen rejection unit.

14. The method as claimed inclaim 13 wherein said impurities are selected from the group consisting of water, carbon dioxide, non-methane organic compounds and sulfur compounds.

15. The method as claimed inclaim 13 wherein nitrogen is recovered from said nitrogen rejection unit.

16. The method as claimed inclaim 13 wherein the pressure in said liquefier heat exchanger range from 6 to 30 bar.

17. The method as claimed inclaim 13 wherein said liquefied natural gas is expanded to a pressure of 1 to 5 bar.

18. The method as claimed inclaim 13 wherein said flash gas is richer in nitrogen than natural gas.

19. The method as claimed inclaim 13 wherein said natural gas stream is selected from the group consisting of landfill gas and biogas.

20. The method as claimed inclaim 13 wherein said flash gas is recycled to said natural gas feed.

21. The method as claimed inclaim 13 wherein flash pressure is lower than storage pressure.

22. The method as claimed inclaim 13 wherein said nitrogen rejection unit is a vacuum swing adsorption unit.

23. The method as claimed inclaim 13 wherein said recovered natural gas is fed to a storage unit.

24. The method as claimed inclaim 13 wherein said storage unit is situated horizontally.

25. The method as claimed inclaim 13 wherein said recovered refrigeration provides cooling to a heat exchanger.

26. The method as claimed inclaim 13 wherein said heat exchanger is in thermal contact with the liquefier feed.

27. An apparatus comprising a nitrogen rejection unit, a liquefier heat exchanger and a flash vessel.

28. The apparatus as claimed inclaim 27 wherein said nitrogen rejection unit is a vacuum swing adsorption unit.

29. The apparatus as claimed inclaim 27 wherein said liquefier heat exchanger is in thermal communication with said flash vessel.

30. The apparatus as claimed inclaim 27 wherein said nitrogen rejection unit is in thermal communication with said liquefier heat exchanger.

31. The apparatus as acclaimed inclaim 27 wherein said flash vessel is in fluid communication with a storage tank.

32. The apparatus as claimed inclaim 27 further comprising a second heat exchanger.

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