Abstract:
An apparatus and process for removing natural gas liquids from an liquid natural gas (LNG) stream, specifically recovering at least 90% of ethane from the LNG, utilizing the low-level heat available in an LNG terminal.

Description:
FIELD OF THE INVENTION 
     The invention pertains to efficient recovery of natural gas liquids, particularly ethane, from liquid natural gas. 
     BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic representation of one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a schematic representation of an embodiment of the invention and depicting its process is shown. An LNG processor  10  comprises an LNG inlet  12 . LNG from LNG inlet  12  is fed to LNG feed pump  14 , from which the LNG passes to feed preheater  16 . Flow of the LNG into the LNG processor  10  is managed by flow controller  18 , which detects flow of LNG and controls flow valve  20 . The LNG inlet stream  22  then passes through heat exchanger  24 , which provides heat exchange between the LNG inlet stream  22  and the LNG product stream  26 , condensing the LNG product stream  26 , and partially vaporizing the LNG inlet stream  22 . Heat exchanger  24  may be a brazed aluminum type heat exchanger, a shell and tube heat exchanger, or another type of heat exchanger appropriate to the application. 
     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.