Patent Grant
7530236
The invention pertains to efficient recovery of natural gas liquids, particularly ethane, from liquid natural gas.
Natural gas is often transported in the form of liquid natural gas (“LNG”). LNG usually is primarily composed of methane, but also comprises lesser amounts of heavier hydrocarbons, such as ethane, propane, and heavier hydrocarbons. When LNG is off-loaded at a terminal, it is often desirable to separate ethane or other heavier hydrocarbons from the LNG in the form of natural gas liquids (“NGL”). The separated NGL can be treated as Y-grade product, or can be further processed in a deethanizer to produce both an ethane product and a Y-grade product.
However, it is also desirable to reduce the power consumption involved in separating NGL from LNG. This process typically requires a flash absorber and a demethanizer. LNG terminals will typically have low level heat available for reboiling, therefore it is desirable to operate the demethanizer at a lower than normal pressure to efficiently utilize the available energy.
It is desirable to use a cooled portion of the LNG product stream to reflow to the flash absorber. Additionally, to operate the demethanizer at a lowered pressure, it is desirable to use a portion of the LNG product stream to reflow to the demethanizer to aid in the removal of NGL from the LNG. Demethanizer feed preheaters and side reboilers are desirably utilized to reduce the duty cycle of the demethanizer bottom reboiler.
It is an object of the invention to separate NGL from LNG in a cost-effective process.
It is a further object of the invention to separate NGL from LNG using a demethanizer at a relatively low pressure.
It is yet another object of the invention to efficiently recover at least 90% of the ethane from a LNG feed.
The invention is an apparatus and process for separating NGL from LNG at a LNG terminal, in particular allowing recovery of at least 90% of the ethane from the LNG feed in a cost-efficient manner. At the process inlet, the LNG feed is preferably routed through a heat exchanger for use in condensing the LNG product stream. This heat exchanger may be partially bypassed, if necessary. The LNG feed is then delivered to a flash absorber allowing initial separation of NGL, particularly ethane, from the LNG feed. The bottom product of the flash absorber is removed, heated, and routed into a demethanizer. Overhead product from the flash absorber is compressed and becomes one part of the LNG product stream.
Heating the bottom product from the flash absorber and use of a side reboiler in the demethanizer provides greater energy efficiency and reduction of the duty cycle of the demethanizer bottom reboiler from what would be otherwise required. Bottom product of the demethanizer is the NGL product stream, which can be further processed in a deethanizer to produce an ethane product stream, or simply be metered out as Y-grade product.
Overhead product of the demethanizer is combined with the compressed overhead product of the flash absorber to form the LNG product stream. This LNG product stream is condensed, preferably by heat exchange with the LNG feed, as discussed above. A first portion of the LNG product stream is cooled and used as a first reflow stream into the flash absorber. A second portion of the LNG product stream is optionally cooled and used as a second reflow stream into the demethanizer. Use of these first and second reflow streams increase the separation of NGL from the LNG, and allow the demethanizer to be operated at a reduced pressure from what would otherwise be required.
By utilizing the first and second reflow streams, recovery of at least 90% of the ethane in the LNG feed is possible in a cost-effective and energy-efficient manner.
Referring to
On exiting heat exchanger 24, the LNG inlet stream passes to the flash absorber 32. Bypass valve 28 is controlled by temperature controller 30, maintaining the appropriate temperature of inlet stream 22 as it feeds into flash absorber 32. Flash absorber 32 preferably operates at approximately 195 to 210 psia and within a temperature range of −160 to −175° F.
Absorber bottom product 34 from the flash absorber 32 contains a high proportion of NGL, particularly ethane, relative to the content of the LNG inlet stream 22. Absorber bottom product 34 is fed to the demethanizer feed pump 36, which pumps absorber bottom product 34 through the demethanizer feed preheater 38 and into demethanizer 40. Demethanizer 40 operates in a range of approximately 270 to 290 psia at top and temperature ranges of approximately −130 to −165° F. at top and +18 to +65° F. at bottom. Bottom temperature in demethanizer 40 is controlled by regulating side reboiler 68 and bottom reboiler 70. Typical heat sources available for such reboilers in LNG terminals is in the operating temperature range of approximately 70 to 90° F. The heating media can be a mixture of ethylene glycol and water, propylene glycol and water, or other combinations of heating fluids commercially available and known to those skilled in the art.
Absorber overhead product 42 from the flash absorber 32 is compressed by compressor 44 to within a range of approximately 280-300 psia. Compressor 44 is controlled by pressure controller 46, which controls the pressurization control 48 on the compressor 44. Compressed absorber overhead product 50 from the flash absorber 32 is joined with demethanizer overhead product 52 to form LNG product stream 26. As discussed above, LNG product stream 26 is passed through heat exchanger 24 and condensed. Condensed LNG product stream 54 feeds into LNG surge drum 56 at LNG product outlet 55. LNG surge drum 56 operates in a pressure range of approximately 260 to 280 psia and a temperature range of approximately −180 to −195° F. A first portion 58 of condensed LNG product stream 54 may be cooled by feed preheater 16. The cooled first portion 60 feeds into flash absorber 32 to increase absorption and separation of heavier hydrocarbons, particularly ethane, thus increasing the ethane concentration of the bottom product and increasing the methane concentration of the overhead product.
A second portion 62 of condensed LNG product stream 54 is pumped by LNG reflow pump 64, and may optionally be cooled by LNG reflow cooler 66, then feeds into demethanizer 40. LNG reflow cooler 66 may be used as a matter of convenience, but demethanizer feed preheater 38 and heat exchanger 24 may be utilized to provide adequate control of temperature conditions in demethanizer 40. As with the LNG reflow in flash absorber 32, use of LNG reflow in demethanizer 40 increases heavier hydrocarbon concentration in the demethanizer bottom product 72 and increases methane concentration in demethanizer overhead product 52 while allowing demethanizer 40 to operate at lower pressures than would otherwise be required. Side reboiler 68 and demethanizer feed preheater 38 allow reduced duty cycles in bottom reboiler 70.
Alternatively, reflow of either the first portion 58 or the second portion 62 of the condensed LNG product stream 54 may be omitted. If reflow of the first portion 58 of the condensed LNG product stream 54 is omitted, there will be no reflow to the flash absorber 32. If reflow of the second portion 62 of the condensed LNG product stream 54 is omitted, there will be no reflow to the demethanizer 40. In either case, the overall efficiency of the NGL recovery would be lessened relative to that available by using both reflows.
Demethanizer bottom product 72 may be pumped by NGL product pump 74 to NGL terminal 76 for delivery as Y-grade NGL product. Similarly, LNG product 78 is delivered to LNG terminal 80. An alternative treatment of the NGL product may be provided by delivering demethanizer bottom product 72 to a deethanizer 82 (details not shown) and using ethane delivery pump 84 and NGL delivery pump 88 to deliver ethane and Y-grade NGL product to ethane terminal 86 and Y-grade NGL product terminal 90 respectively.
Those of skill in the art will recognize that heat exchange relationships may be altered in the above description without departing from the spirit of the invention, so long as flow streams are heated and cooled appropriately to maintain desired operating conditions. Accordingly, selection of various flow streams used in heat exchangers as described above is a matter of engineering preference and efficiency.
The above examples are included for demonstration purposes only and not as limitations on the scope of the invention. Other variations in the construction of the invention may be made without departing from the spirit of the invention, and those of skill in the art will recognize that these descriptions are provide by way of example only.
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| Number | Date | Country | |
|---|---|---|---|
| 20070204651 A1 | Sep 2007 | US |
