Patent Publication Number: US-2017370641-A1

Title: Systems and methods for removal of nitrogen from lng

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable. 
     FIELD OF THE INVENTION 
     The subject matter disclosed herein generally relates to systems and methods for the removal of nitrogen from liquid natural gas (herein, “LNG”), particularly, LNG produced from LNG liquefaction plant, more particularly, LNG having a nitrogen content of from about 4 mole % to about 15 mole %. 
     BACKGROUND OF THE INVENTION 
     Natural gas is produced from various geological formations. Natural gas produced from various geological formations typically contains methane, ethane, propane, and heavier hydrocarbons, as well as trace amounts of various other gases, such as nitrogen, carbon dioxide, and hydrogen sulfide. The various proportions of these components may vary, for example, depending upon the geological formation from which the natural gas is produced. 
     The nitrogen content of natural gas varies depending on the characteristics of the gas fields from which the natural gas is produced. For example, in some low yield oil fields, nitrogen may be injected to enhance oil production, which thereby may lead to an increase in nitrogen content of the hydrocarbons produced from those fields over time. For example, the nitrogen content of the hydrocarbons produced from a given oil field may initially range from 1 to 2 mole %, but as the fields mature, and with continuous nitrogen injection, the nitrogen content can increase to as high as 15 mole % of the produced hydrocarbons. 
     LNG having a relatively-high nitrogen content may be undesirable for several reasons. For example, increases in nitrogen content of the LNG are associated with decreases in the energy content (e.g., BTU value) of the LNG product and increases in the transportation costs for the LNG, in that the nitrogen (which is an inert) must be transported along with the LNG. Further, the boil off gas (herein, “BOG,” which refers to LNG that becomes vaporous, for example, due to ambient heat and/or changes in pressure) is also enriched in nitrogen, thereby making the BOG unsuitable for combustion. Further still, relatively-high nitrogen content LNG may not be compatible with relatively-low nitrogen content LNG in the storage tanks; for example, mixing or intermingling high and low nitrogen content LNG within a storage tank may lead to stratification and/or rollover of the LNG within the tank, thereby leading to problems resulting from the mixing of dissimilar LNGs, and creating safety concerns. 
     Conventionally, LNG plants can accept an LNG feed liquid with relatively-low nitrogen content (e.g., up to about 3 mole %) without necessitating specialized processing equipment. LNG plants are generally limited to accepting relatively-low nitrogen content LNG by the fact that such conventional LNG plants are limited as to the proportion of nitrogen that can be removed from an LNG product, for example, via a flash-drum (e.g., only about 75% of the nitrogen present) at the end of the process or system. 
     As such, in order to process an LNG feed gas having a relatively-high nitrogen content (e.g., about 4 mole % nitrogen, or more), a nitrogen rejection unit (NRU) has conventionally been required to be installed upstream of the LNG plant, for example, in order to meet the 1 mole % nitrogen specification in LNG. Depending on the feed gas composition and flow rate, the NRU might have employed, for example, a membrane separator, molecular sieves, or cryogenic fractionator to separate nitrogen from the LNG feed gas and reject the nitrogen to the atmosphere, thereby lowering the nitrogen content of the LNG feed gas. 
     However, the use of such a cryogenic fractionator is energy intensive and requires high capital inputs and operating costs, which can seldom be justified for LNG production and processing. While membranes and molecular sieves can also remove nitrogen from an LNG feed gas, the use of membranes and molecular sieves is generally limited to small-throughput systems or units. For instance, membranes typically produce a waste nitrogen stream with a high hydrocarbon content, which is a revenue loss and cannot be vented to the atmosphere, as it is a source of greenhouse gas(es). When membranes are employed, waste nitrogen must either be reinjected for sequestration or disposed by other means. Molecular sieves are also uneconomical when used to remove high levels of nitrogen. Thus, although various NRUs may be used to reduce the nitrogen content of an LNG feed gas, those NRUs suffer from the disadvantages, such as high cost and process complexity, especially where the feed gas has a relatively-high nitrogen content (e.g., greater than 4 mole %) and, as such, conventional LNG plants are poorly suited for processing relatively-high nitrogen content LNG. 
     Therefore, there is a need for methods and systems of removing nitrogen from LNG such that relatively-high ogee content LNG can be efficiently processed with lessened cost and complexity than that of conventional processing systems and methods. 
     SUMMARY OF THE INVENTION 
     Disclosed herein are systems (e.g., plant configurations) and methods for removing nitrogen from a LNG product stream having a relatively-high nitrogen content (for example, with a nitrogen content of up to 15 mole %). In some embodiments, the disclosed systems and methods may remove about 95% of the nitrogen present in a LNG product stream. In some embodiments, the disclosed systems and methods may be located downstream relative to a LNG liquefaction plant. For example, such systems and methods can be used in a “grass-root” LNG plant (e.g., a plant located near a field from which the gas being processed is taken) to process a relatively-high nitrogen content LNG from a nearby field. Also, such systems and methods can be employed to reconfigure (e.g., as an add-on of one or more units or components) for revamping an existing facility. 
     In an embodiment, the disclosed system comprises a feed exchanger that is fluidly coupled with a stripper. The stripper produces a side-draw that is used to chill the LNG feed stream and to strip nitrogen content from the LNG feed stream. 
     In an embodiment, the stripper comprises a chimney tray configured to draw liquid from the second upper-most tray of the stripper, alternatively, the third upper-most tray of the stripper, at an elevation that is at least about 15 feet higher than the heat exchanger. In such an embodiment, the chimney tray may yield sufficient hydraulic head to operate the thermosiphon process. 
     In an embodiment, the side-draw liquid is controlled to maintain a molar flow ratio of the side-draw to the LNG feed of about 0.30 to 0.70. The side-draw may be heated by the LNG feed stream to form a stripping medium that is routed to a lower section of the stripper, while the LNG feed is cooled by the side-draw prior to being let down in pressure and further chilled, thereby producing a cold reflux to the stripper for fractionation. 
     Disclosed herein is a system for the removal of nitrogen from a liquid natural gas (LNG) stream. The system comprises a feed heat exchanger. The heat is configured to receive the LNG stream and to cool the LNG stream via heat exchange with a stripper column side-draw stream to yield a cooled LNG stream and a heated side-draw stream. The system also comprises a stripper column. The stripper column is configured to receive the cooled LNG stream at a first tray and the heated side-draw stream. The stripper column is configured to produce the stripper column side-draw stream, a stripper column overhead stream, and a stripper column bottom stream. The stripper column is configured such that the stripper column side-draw stream is taken from the stripper column at a second tray. The second tray is at least about 15 feet higher than the feed heat exchanger. 
     Also disclosed herein is a method for the removal of nitrogen from a liquid natural gas (LNG) stream. The method comprises receiving, at a feed heat exchanger, the LNG stream. The method also comprises cooling the LNG stream via heat exchange with a stripper column side-draw stream to yield a cooled LNG stream and a heated side-draw stream. The method also comprises receiving, at a stripper column, the cooled LNG stream at a first tray and the heated side-draw stream. The method also comprises producing the stripper column side-draw stream from the stripper column at a second tray that is at least about 15 feet higher than the feed heat exchanger. The method also comprises producing a stripper column overhead stream and a stripper column bottom stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an embodiment of the nitrogen removal system. 
         FIG. 2  illustrates the heat exchange curves for the feed heat exchanger of  FIG. 1 . 
         FIG. 3  depicts the nitrogen removed via operation of the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein are systems and methods for removing nitrogen from a LNG product stream using a specially-configured stripper and feed heat exchanger. In some of the disclosed embodiments, the removal of nitrogen from the LNG product stream is accomplished without the use of external heating and cooling, for example, thereby increasing the heating value of the resultant LNG product stream. 
     Referring to  FIG. 1 , an embodiment of the disclosed nitrogen removal (NR) system  100  is illustrated. In the embodiment of  FIG. 1 , the NR system  100  generally comprises a feed heat exchanger  56 , a first valve  57 , a second valve  58 , and a stripper column  59 . 
     In the embodiment of  FIG. 1 , the NR system  100  is shown in operation. In operation, a LNG feed liquid stream  4  is introduced into the feed heat exchanger  56 . The LNG feed liquid stream  4  may be from an LNG liquefaction plant, for example, of which the NR system may be a sub-system. 
     In such an embodiment, a gas stream, which may have a pressure of about 850 psig, may be treated in an acid gas removal unit to decrease the acid gas content thereof, for example, such that the gas stream contains no more than 50 ppmv CO 2  and 4 ppmv H 2 S. The treated gas stream may be dried, for example, in a molecular sieve dehydration unit to thereby produce a dry gas stream. If mercury is present in the feed gas, the mercury may also be removed, for example, to lessen the potential for mercury corrosion of any process equipment. This purified gas stream may then be fed to the LNG liquefaction plant, which produces the LNG feed stream  4  (e.g., a high-pressure subcooled LNG stream). 
     The LNG feed liquid stream  4  may have a temperature and pressure of about −238° F. and about 850 psig; for example, the LNG feed liquid stream  4  may be characterized as a high-pressure LNG stream. The LNG liquid gas stream  4  may be characterized as a relatively-high nitrogen content LNG stream, for example, having about 4% nitrogen content, alternatively, about 5%, alternatively, about 6%, alternatively, about 7%, alternatively, about 8%, alternatively, about 9%, alternatively, about 10%, alternatively, about 11%, alternatively, about 12%, alternatively, about 13%, alternatively, about 14%, alternatively, up to about 15 mole % nitrogen content. 
     The LNG feed liquid stream  4  (e.g., a high-pressure LNG stream) is cooled in the feed heat exchanger  56  by a letdown stripper column side-draw stream  10  to yield a subcooled LNG stream  7 . The subcooled LNG stream  7  may have a temperature of from about −256° F. to −275° F. 
     Referring to  FIG. 2 , the heat exchange curves for the feed heat exchanger  56  are shown. In an embodiment, the feed heat exchanger  56  is configured to operate with a close temperature approach, for example, thereby minimizing thermodynamic losses. In an embodiment, the feed heat exchanger  56  is a brazed aluminum exchanger, which can achieve close temperature approaches. Not intending to be bound by theory, as can been seen in  FIG. 2 , the pinch point of the heat exchange curves is in the mid-section where vaporization of the side-draw liquid occurs. 
     Referring again to  FIG. 1 , the subcooled LNG stream  7  is passed through the second valve  58  (e.g., a JT valve), for example, to reduce (e.g., decrease or “let down”) the pressure of the subcooled LNG stream  7  and thereby yield a two-phase stream  8 . In an embodiment, passing the subcooled LNG stream  7  through the second valve  58  to thereby form the two-phase stream  8  may further cool or subcool the LNG, for example, to a temperature of about −262° F. to −287° F. Not intending to be bound by theory, passing the subcooled LNG stream  7  through the second valve  58  may help to suppress the temperature thereof, for example, to suppress the temperature of the subcooled LNG stream  7  to a preferred lower temperature, for example, about −238° F. to −275° F. In alternative embodiments, any suitable pressure reduction device may be employed in place of the second valve  58 . 
     in the embodiment of  FIG. 1 , the two-phase stream  8  is fed to the stripper column  59 , which may comprise a plurality of trays. In the embodiment of  FIG. 1 , the two-phase stream  8  is fed into the stripper column  59  at a first tray, for example, a two-phase distributor tray  60  configured to separate, alternatively, to substantially separate, the two phases of the two-phase stream  8 , for example, to separate the flash vapor component (e.g., BOG) from the liquid component. In an embodiment, the two-phase stream  8  is fed into the stripper column  59  at an upper section of the stripper column  59 ; for example, the first tray to which the two-phase stream  8  is fed may be in an upper (e.g., an upper-most) section of the stripper column. 
     In the embodiment, the stripper column  59  is operated to yield a stripper column side-draw stream  9 , a stripper column overhead vapor stream  6 , and a stripper column bottom stream  5 . The stripper column side-draw stream  9  is withdrawn from a second tray of the stripper column, for example, a chimney tray  61 . In an embodiment, the second tray (e.g., the chimney tray) is configured to draw a liquid from the stripper column  59 . In an embodiment, the chimney is the second upper-most tray of the stripper column  59 , alternatively, the third upper-most tray of the stripper column  59 , alternatively, the fourth or, alternatively, a higher upper-most tray of the stripper column  59 . Additionally or alternatively, in an embodiment, the tray from which the stripper column side-draw stream  9  is withdrawn is at least about 15 feet higher than the feed heat exchanger  56 , alternatively, at least about 16 feet, alternatively, at least about 17 feet, alternatively, at least about 18 feet, alternatively, at least about 19 feet, alternatively, at least about 20 feet, alternatively, at least about 22 feet, alternatively, at least about 24 feet higher than the feed heat exchanger. Not intending to be bound by theory, the elevation at which the stripper column side-draw stream  9  is withdrawn may be sufficient to provide sufficient head to operate the thermosiphon system hydraulic. 
     Referring again to  FIG. 1 , the stripper column side-draw stream  9  is passed through the first valve  57 , for example, to control flow rate of the stripper column side-draw stream  9  while maintaining the level in the chimney tray  61  for stability, and thereby yield the letdown stripper column side-draw stream  10 . In alternative embodiments, any suitable controlling device may be employed in place of the first valve  57 . The letdown stripper column side-draw stream  10  may have a temperature of from about −262 to −280° F. 
     In the embodiment of  FIG. 1 , the letdown stripper column side-draw stream  10  is introduced into the feed heat exchanger  56 , for example, to cool the LNG feed liquid stream  4  as discussed herein, thereby forming a heated LNG stream  11 . The heated LNG stream  11  may be characterized as a two-phase stream. 
     In the embodiment of  FIG. 1 , the heated LNG stream  11  is fed (e.g., returned) to the stripper column  59 . In an embodiment, the heated LNG stream  11  is fed to the second lower-most tray, alternatively, the third lower-most tray, alternatively, the fourth lower-most tray or, alternatively, a higher tray above the bottom section of the stripper column  59 . Not intending to be bound by theory, introducing the heated LNG stream  11  to such a relatively-lower tray may ensure sufficient contact and mixing with the various tray liquids of the stripper column. 
     The stripper column overhead vapor stream  6  produced by the stripper column may be characterized as having a nitrogen content of from about 35 mole % to 80 mole %. The stripper column bottom stream  5  may be characterized as having a nitrogen content of from about 0.2 to 0.9 mole %. For example, the LNG yielded via the stripper column bottom stream  5  may be suitable for LNG export or storage. The high nitrogen content pertains to LNG feed stream with 15 mole % while the low nitrogen content pertains to the 4 mole % LNG feed. 
     Referring to  FIG. 3 , the nitrogen removal resulting from the operation of the system of  FIG. 1  is shown. As shown in  FIG. 3 , when the stream entering the process (e.g., LNG feed liquid stream  4 ) has a nitrogen content of about 4 mole %, the disclosed system and process can produce an LNG product stream (e.g., the stripper column bottom stream  5 ) with about 0.2 mole % nitrogen, which is significantly lower than would have been achieved via conventional systems and methods utilizing a flash-drum at the end of such conventional process, which would be about 1 mole %. As also shown in  FIG. 3 , when the stream entering the process (e.g., LNG feed gas stream  4 ) has a nitrogen content of about 15 mole %, where a conventional end-of-process flash-drum is unable to remove a sufficient amount of nitrogen, the disclosed system and process can produce an LNG with less than 0.9 mole % nitrogen. 
     Further, in addition to yielding an LNG product meeting desired LNG specifications, the disclosed systems and processes also yield an LNG product having an improved heating value, for example, the heating value of the LNG product may be improved by from about 5 to 17%, depending on the nitrogen content of the LNG feed. As such, the disclosed systems and processes are particularly useful for processing natural gas in existing facilities, for example, in order to meet nitrogen specifications, from fields yielding natural gas with an increasing nitrogen content. As such, the disclosed systems and processes are cost-effective and are capable of achieving nitrogen removal from an LNG stream, maximizing the heating value of such LNG stream, increasing the value of the LNG product. 
     Additional Embodiments 
     A first embodiment, which is a system for the removal of nitrogen from a liquid natural gas (LNG) stream, the system comprising a feed heat exchanger, wherein the heat is configured to receive the LNG stream and to cool the LNG stream via heat exchange with a stripper column side-draw stream to yield a cooled LNG stream and a heated side-draw stream; and a stripper column, wherein the stripper column is configured to receive the cooled LNG stream at a first tray and the heated side-draw stream, and wherein the stripper column is configured to produce the stripper column side-draw stream, a stripper column overhead stream, and a stripper column bottom stream, wherein the stripper column is configured such that the stripper column side-draw stream is taken from the stripper column at a second tray, wherein the second tray is at least about 15 feet higher than the feed heat exchanger. 
     A second embodiment, which is the system of the first embodiment, further comprising a pressure reduction device, wherein the pressure reduction device is configured to reduce the pressure of the stripper column side-draw stream prior to introduction into the feed heat exchanger. 
     A third embodiment, which is the system of the second embodiment, wherein the pressure reduction device is a valve. 
     A fourth embodiment, which is the system of one of the second through the third embodiments, further comprising a control device, wherein the control device is configured to control the flow of the heated side-draw stream prior to introduction into the feed heat exchanger. 
     A fifth embodiment, which is the system of the fourth embodiment, wherein the control device is a flow control valve. 
     A sixth embodiment, which is the system of one of the first through the fifth embodiments, wherein the system is configured to receive the LNG stream from a LNG liquefaction plant. 
     A seventh embodiment, which is the system of one of the first through the sixth embodiments, wherein the LNG stream is characterized as a relatively-high nitrogen content LNG stream. 
     An eighth embodiment, which is the system of the seventh embodiment, wherein the stripper column bottom stream has less than about 0.9 mole % nitrogen. 
     A ninth embodiment, which is the system of one of the first through the eighth embodiments, wherein the cooled LNG stream has a temperature of from about −256° F. to −275° F. 
     A tenth embodiment, which is the system of one of the first through the ninth embodiments, wherein the second tray is at least about 20 feet higher than the feed heat exchanger. 
     An eleventh embodiment, which is a method for the removal of nitrogen from a liquid natural gas (LNG) stream, the method comprising receiving, at a feed heat exchanger, the LNG stream, cooling the LNG stream via heat exchange with a stripper column side-draw stream to yield a cooled LNG stream and a heated side-draw stream, receiving, at a stripper column, the cooled LNG stream at a first tray and the heated side-draw stream, producing the stripper column side-draw stream from the stripper column at a second tray that is at least about 15 feet higher than the feed heat exchanger; and producing a stripper column overhead stream and a stripper column bottom stream. 
     A twelfth embodiment, which is the method of the eleventh embodiment, further comprising controlling the flow of the stripper column side-draw stream prior to introducing the stripper column side-draw stream into the feed heat exchanger. 
     A thirteenth embodiment, which is the method of the twelfth embodiment, wherein controlling the flow of the stripper column side-draw stream comprises passing the stripper column side-draw stream through a first valve. 
     A fourteenth embodiment, which is the method of one of the eleventh through the thirteenth embodiments, further comprising controlling the flow of the heated side-draw stream prior to introducing the heated side-draw stream into the feed heat exchanger. 
     A fifteenth embodiment, which is the method of the fourteenth embodiment, wherein controlling the flow of the heated side-draw stream comprising passing the heated side-draw stream through a second valve. 
     A sixteenth embodiment, which is the method of one of the eleventh through the fifteenth embodiments, wherein receiving the LNG stream comprises receiving the LNG stream from a LNG liquefaction plant. 
     A seventeenth embodiment, which is the method of one of the eleventh through the sixteenth embodiments, wherein the LNG stream is characterized as a relatively-high nitrogen content LNG stream. 
     An eighteenth embodiment, which is the method of the seventeenth embodiment, wherein the stripper column bottom stream has less than about 0.9 mole % nitrogen. 
     A nineteenth embodiment, which is the method of one of the eleventh through the eighteenth embodiments, wherein cooling the LNG stream comprises cooling the LNG stream to a temperature of from about −256° F. to −27.5° F. 
     A twentieth embodiment, which is the method of one of the eleventh through the nineteenth embodiments, wherein the second tray is at least about 20 feet higher than the feed heat exchanger.