Nitrogen Rejection Unit with Mixed Refrigerant Cooling System and Method

Description

CLAIM OF PRIORITY

This application claims the benefit of U.S. Provisional Application No. 63/724,532, filed Nov. 25, 2024, the contents of which are hereby incorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to a natural gas liquefaction system including a nitrogen rejection unit (NRU) for the removal of nitrogen from a natural gas stream. In particular, the present disclosure relates to a natural gas liquefaction system and method including a nitrogen rejection unit and a mixed refrigerant cooling system.

BACKGROUND

Typically, natural gas includes an amount of nitrogen. For example, natural gas may include naturally occurring nitrogen in the range from 0-5 mol. %. In some instances, natural gas fields can include more than 20 mol. % nitrogen.

Many natural gas pipelines have nitrogen specifications in the range of 1-3 mol. %. Accordingly, natural gas with a high nitrogen content may require removal of the nitrogen prior to or during the natural gas liquefaction process whereby liquid natural gas (LNG) is created to meet product specification and to ensure safe transport and/or storage. Typically, nitrogen content in LNG is maintained below 1 mol. % to increase the heating value of the transported LNG.

Traditionally, the separation of nitrogen from natural gas has been achieved through various methods such as an end flash step, where high pressure LNG from a liquefaction unit is reduced in pressure to generate vapor. The end flash step can utilize an end flash drum or a nitrogen stripping column. Additional methods include cryogenic distillation, pressure swing adsorption, and membrane separation. These methods often entail high capital expenditure (CAPEX), require significant energy consumption, and may not always be suitable for processing natural gas streams with varying compositions and operating conditions.

Standalone nitrogen rejection units (NRUs) may be used to further process feed streams or flash gas. NRUs can produce a nitrogen stream and a nitrogen depleted gas stream containing as little as 1 mol. % nitrogen. The nitrogen removed from the feed stream may be used as fuel or in other applications, sent to flare or vented to atmosphere. Despite being able to produce a purified nitrogen stream, many NRUs require additional equipment such as compressors, distillation columns, separators, and heat exchangers resulting in a high CAPEX.

SUMMARY OF THE DISCLOSURE

There are several aspects of the present subject matter which may be embodied separately or together in the methods, devices and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto.

In one aspect, a system for liquefying a natural gas feed stream is provided. The system includes a liquefaction system including a main heat exchanger configured to cool the natural gas feed stream in a cooling passage. A nitrogen rejection unit is in fluid communication with the cooling passage of the main heat exchanger. The system also includes a mixed refrigerant cooling system associated with the liquefaction system. The mixed refrigerant cooling system is configured to cool the cooling passage of the main heat exchanger of the liquefaction system.

In another aspect, a method of liquefying a natural gas feed stream is provided. The method includes the steps of cooling the natural gas feed stream within a cooling passage of a main heat exchanger to produce a cooled natural gas fluid stream; separating the cooled natural gas fluid stream in a nitrogen rejection unit to produce a liquid natural gas product stream and a first nitrogen vapor stream; cooling a second nitrogen vapor stream from the nitrogen rejection unit in a reflux condenser to form a reflux stream; directing the reflux stream to the nitrogen rejection unit; and cooling the cooling passage of the main heat exchanger with a mixed refrigerant cooling system associated with the liquefaction system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of a natural gas liquefaction system of the present disclosure.

FIG. 2 is a schematic illustration of another embodiment of a natural gas liquefaction system of the present disclosure.

FIG. 3 is a schematic illustration of another embodiment of a natural gas liquefaction system of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

A more detailed description of the system and method in accordance with the present disclosure is set forth below. It should be understood that the description below of specific systems and methods is intended to be exemplary, and not exhaustive of all possible variations or applications. Thus, the scope of the disclosure is not intended to be limiting and should be understood to encompass variations or embodiments that would occur to persons of ordinary skill.

It should be noted herein that the lines, conduits, piping, passages and similar structures and the corresponding streams are sometimes both referred to by the same element number set out in the figures.

The term “column” as used below means a distillation, fractionation or rectification column including a contacting column or zone wherein countercurrent liquid and vapor phases are contacted to cause separation of a fluid mixture such as by contacting the vapor and liquid phases on a series of vertically spaced plates or trays or packing material positioned within the column.

Also, as used herein, and as known in the art, a heat exchanger is that device or an area in the device wherein indirect heat exchange occurs between two or more streams at different temperatures, or between a stream and the environment. In addition, all heat exchangers referenced herein may be incorporated into one or more heat exchanger devices or may each be individual heat exchanger devices.

As used herein, the terms “communication”, “communicating”, and the like generally refer to fluid communication unless otherwise specified. And although two fluids in communication may exchange heat upon mixing, such an exchange would not be considered to be the same as heat exchange in a heat exchanger, although such an exchange can take place in a heat exchanger.

As used herein, the terms, “high”, “middle”, “warm”, “cold” and the like are relative to comparable streams, as is customary in the art.

Reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures for shared elements or components without additional description in the specification in order to provide context for other features. In the claims, letters are used to identify claimed steps (e.g. a., b. and c.). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which the claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims.

The feed stream for a liquefaction system 10 includes a natural gas feed. Typically, natural gas contains nitrogen and methane and may also contain a number of other species which normally occur in natural gas reservoirs such as carbon dioxide and higher hydrocarbons having two to four carbon atoms. Generally, this stream will have been pretreated to remove, to the extent practical, higher hydrocarbons and natural gas liquids. The feed stream may contain 4-7 mole % nitrogen and is typically available at about 200-290 psig. The system 10 may produce a final liquid natural gas product with about 2.0-3.0 mole % nitrogen and a nitrogen vapor with about 0.5-2.5 mole % methane, suitable for venting into the atmosphere.

Turning to FIG. 1, a natural gas feed stream 8 enters a liquefier. For instance, the natural gas stream 8 enters a liquefaction cold box 11 including a series of heat exchangers, expansion devices/valves, separators, and a Nitrogen Rejection Unit (NRU). The natural gas feed stream 8 can be directed to multiple liquefiers. In an embodiment, the liquefiers can all be the same as described herein or may vary.

As shown in FIG. 1, natural gas stream 8 enters the liquefaction cold box 11 and travels through expansion device 9 and into the main heat exchanger 12. As in the case of all expansion devices referenced below, expansion device 9 may be an expansion valve, such as a Joule-Thomson valve, or another type of expansion device including, but not limited to, a turbine or an orifice. In an embodiment, main heat exchanger 12 may be a multi-stream brazed aluminum main heat exchanger (BAHX). Other types of heat exchangers may be used without departing from the scope of the disclosure. The natural gas feed stream 8 is cooled within cooling passage 13 within the heat exchanger 12. In an embodiment, a portion of the natural gas feed stream 8 may be diverted to travel through expansion valve 15 before being received by cooling passage 13 for temperature control. Diverted feed stream 7 enters main heat exchanger 12 at a location further downstream than where the natural gas feed stream 8 travelling though expansion device 9 enters main heat exchanger 12. The diverted feed stream 7 combines with the feed stream 8 within cooling passage 13 of main heat exchanger 12 and is cooled therein.

Cooled stream 17 may exit main heat exchanger 12 and reenter main heat exchanger 12 to be further cooled. Liquid natural gas stream 14 exits the main heat exchanger 12, travels through expansion device 19 and to cooling passage 21 within reboiler heat exchanger 16. The resulting cooled liquid stream 18 passes through expansion device 20 and enters NRU distillation column 22.

In an embodiment, after initially exiting the main heat exchanger 12, cooled stream 17 or a portion thereof, can travel through expansion device 23 and directly into cooling passage 21 within reboiler heat exchanger 16. Additionally, a portion of the cooled stream 17 can bypass reboiler heat exchanger 16 by travelling through valve 25 to rejoin cooled liquid stream 18 downstream valve 20 to provide temperature. The combined cooled liquid stream then enters NRU distillation column 22.

NRU distillation column 22 can be a tray column or a packed column. In some embodiments, as shown in FIGS. 1-3, NRU distillation column 22 can include a tray column section 27 and a packed column section 29. For instance, NRU distillation column can include a packed column section 29 below where the LNG enters NRU distillation column 22. In an embodiment, the packed column section 29 can be about 14 feet long. The column can also include a tray column section 27 above where LNG inlet. In an embodiment, the tray column section 27 can be about 14 feet long. Other NRU distillation column arrangements can be used, such as a stripping column, without departing from the scope of the disclosure.

Liquid and vapor phases are separated within column 22. A portion of the vapor flow in the column 22 is withdrawn as stream 24 from a side vapor outlet port near the top of column 22. Stream 24 is cooled in cooling passage 31 and at least partially condensed in reflux condenser 26 so that reflux stream 28 is formed and returned to the top of NRU column 22. Stream 24 is a mixture of components in the column, principally consisting of nitrogen, methane, and any trace low-boiling components (helium, argon, hydrogen, etc.).

A reboiler stream 30 exits a side port near the bottom of NRU column 22. The reboiler stream 30 is expanded in expansion device 32 and is warmed within warming passage 33 in reboiler heat exchanger 16 to provide refrigeration therein. The resulting warmed stream 34 is returned to the bottom of the NRU column 22.

A column overhead vapor stream 46, which consists primarily of nitrogen, exits the top of NRU column 22 and is directed to a passage 35 within the main heat exchanger 12 to provide cooling therein. The resulting warmed stream 48 is sent to flare.

A bottoms liquid stream 36 (which is primarily LNG) exits the NRU column 22 and is cooled in passage 37 in main heat exchanger 12. In an embodiment, the cooled stream 38 travels through valve 39, exiting the liquefaction cold box 11 and travels through valve 41 to be collected or further processed. Optionally, after traveling through valve 39, the separated LNG stream 38 can be combined with other LNG streams 40 from other liquefiers.

Cooling for the liquefaction system 10, including for main heat exchanger 12 and reflux condenser 26, is provided by a mixed refrigerant system 50 associated with the liquefaction system. For instance, cooling for reflux condenser 26 is provided by a mixed refrigerant stream 94, which is provided by cooling a mixed refrigerant overhead vapor stream 88 from a cold vapor separator 86 in heat exchanger 12. Main heat exchanger 12 provides cooling for streams 8, 36, and 88 in addition to the warming of the nitrogen vapor stream 46 from the top of NRU column 22. In an embodiment, a portion of the mixed refrigerant system 50 is located outside of the liquefaction cold box 11. For example, the mixed refrigerant system 50 can include compressor(s), after-coolers, and at least one separator located outside of the liquefaction cold box. In particular, compressor 51, accumulators/drums 62, 76, and 136, and after-coolers 58, 72, and 75 of the mixed refrigerant system 50 can be located outside of the liquefaction cold box 11. Additionally, the mixed refrigerant system 50 can include at least one separator configured to separate mixed refrigerant streams within the liquefaction cold box 11. In particular, separators 86, 98, 112, and 128 can be located within the liquefaction cold box 11. In an embodiment, the mixed refrigerant cooling system 50 includes the mixed refrigerant cooling system as disclosed in U.S. Pat. Nos. 9,441,877 and 11,187,457, each of which are hereby incorporated by reference herein in their entirety.

With reference to the mixed refrigerant system 50, the first stage 52 of a compressor 51 receives a vapor mixed refrigerant stream 54 and compresses it. The resulting stream 56 then travels to a first stage after-cooler, such as condenser 58, where it is cooled and partially condensed. The resulting mixed-phase refrigerant stream 60 travels to an interstage drum 62. In an embodiment, the interstage drum 62 can be a low-pressure accumulator. Within interstage drum 62, mixed-phase refrigerant stream 60 is separated into a vapor stream 64 and high-boiling refrigerant liquid stream 66. While interstage drum 62 can be a low-pressure accumulator, alternative separation devices may be used, including, but not limited to, a standpipe or another type of vessel, a cyclonic separator, a distillation unit, a coalescing separator or mesh or vane type mist eliminator. This applies for all accumulators, separators, separation devices and standpipes referenced herein.

Vapor stream 64 travels from the vapor outlet of interstage drum 62 to the second stage 68 of the compressor 51 where it is compressed to a high pressure. Stream 70 exits the compressor second stage 68 and travels through a second stage after-cooler where it is cooled. In an embodiment, the second stage after-cooler can be a discharge desuperheater 72. The resulting stream 74 travels through another after-cooler, such as a discharge condenser 75. In an embodiment, instead of employing discharge desuperheater 72 and discharge condenser 75, a single after-cooler to condense the stream can be used without departing from the scope of the disclosure. The resulting stream 77 contains both vapor and liquid phases which are separated in accumulator 76. In an embodiment, accumulator 76 can be a high-pressure accumulator. Stream 77 is separated within accumulator 76 to form high-pressure vapor stream 78 and high-pressure refrigerant liquid stream 80.

While the first and second compressor stages 52 and 68 are illustrated as part of a single compressor 51, individual compressors may be used instead. In addition, the system is not limited to solely two compression and cooling stages in that more or less may be used. Furthermore, in an embodiment, as illustrated, compressor 51 can be driven by a gas turbine 53.

Turning to the heat exchanger system, the heat exchanger 12 includes a high-pressure vapor passage 82 which receives the high-pressure vapor stream 78 from the accumulator 76 and cools it so that it is partially condensed. The resulting mixed-phase cold separator feed stream 84 is provided to a cold vapor separator 86 so that cold separator vapor stream 88 and cold separator liquid stream 90 are produced.

The heat exchanger 12 includes a cold separator vapor passage 92 that receives the cold separator vapor stream 88. The cold separator vapor stream 88 is cooled in passage 92 and is condensed into liquid stream 94, flashed through expansion device 96 and directed to cold temperature separator, such as cold-temperature standpipe 98 to form a cold-temperature liquid stream 100 and a cold-temperature vapor stream 102. As disclosed above, in the case of all expansion devices referenced for the mixed refrigerant refrigeration system 50, expansion device 96 may be an expansion valve, such as a Joule-Thomson valve, or another type of expansion device including, but not limited to, a turbine or an orifice. The cold-temperature liquid stream 100 and vapor stream 102 are combined (within the heat exchanger 12, within a header of the heat exchanger 12, or prior to entry into a header of the heat exchanger 12) and directed to primary refrigeration passage 104 to provide cooling within the heat exchanger 12.

In addition to traveling to cold-temperature separator 98, a portion of liquid stream 94 is divided off at a split as stream 42 and travels through expansion device 106 and through passage 108 to cool the mixed stream travelling through passage 31 in reflux condenser 26. The warmed stream 110 exits the reflux condenser 26 and travels to mid-temperature separator, such as mid-temperature standpipe 112.

The cold separator liquid stream 90 is cooled in cold separator liquid passage 114 to form subcooled cold separator liquid 116, which is flashed at expansion device 118 and directed to mid-temperature standpipe 112. The cold separator liquid 116 is combined with the warmed stream 110 within the mid-temperature standpipe 112. A resulting mid-temperature liquid stream 120 and a resulting mid-temperature vapor stream 122 are combined (within the heat exchanger 12, within a header of the heat exchanger 12, or prior to entry into a header of the heat exchanger 12) and directed to the primary refrigeration passage 104 to provide cooling within heat exchanger 12. In such an arrangement, the mid-temperature separator 112 improves thermodynamic and fluid distribution performance.

The liquid stream 80 is directed from the accumulator 76 through a high-pressure liquid passage 124 of the heat exchanger 12. The stream is subcooled and then flashed using expansion device 126 and directed to upper/high-temperature standpipe 128 to form the high-temperature refrigerant vapor stream 130 and high-temperature liquid stream 132, which are combined (within the heat exchanger 12, within a header of the heat exchanger 12, or prior to entry into a header of the heat exchanger 12) and directed to the primary refrigeration passage 104 to provide cooling within heat exchanger 12.

Additionally, high-boiling refrigerant liquid stream 66 from interstage drum 62 travels through high-boiling refrigerant liquid stream passage 67 where it is cooled. The resulting cooled liquid stream is flashed with expansion device 69 and is directed to the primary refrigeration passage 104 to provide cooling within heat exchanger 12.

The combined refrigerant streams from the cold standpipe 98, mid-temperature standpipe 112, high-temperature standpipe 128, and interstage drum 62 exit the primary refrigeration passage 104 as a combined return refrigerant stream 134, which preferably is in the vapor phase. The return refrigerant stream 134 flows to an optional suction drum 136, which results in vapor mixed refrigerant stream 54. The optional suction drum 136 guards against liquid being delivered to the system compressor 51.

Turning to FIGS. 2 and 3, the figures illustrate liquefaction systems 210 and 310, which operate in the same manner as the system disclosed in FIG. 1 with the exception that liquid natural gas 36 from the bottom of NRU distillation column 22 is cooled in heat exchanger 12 to provide cooling for reflux condenser 26. For illustrative purposes, elements of system 210 (as shown in FIG. 2) and system 310 (as shown in FIG. 3) similar to elements of system 10 as shown in FIG. 1 will be referenced with the same reference number, whereas different elements will be referenced with a different reference number.

As shown in FIG. 2, the bottoms liquid stream 36 (which is primarily LNG) exits the NRU column 22 and is cooled in passage 37 in main heat exchanger 12. After the cooled liquid stream 212 exits the heat exchanger 12, it encounters a split whereby a portion 213 of the cooled liquid stream is directed back into the heat exchanger, through cooling passage 214. The cooled stream 216 travels through valve 218, exiting the liquefaction cold box 11 to be collected or further processed. Optionally, after traveling through valve 218, the diverted LNG stream 213 can be combined with LNG stream 226 used to cool reflux condenser 26 and/or with other LNG streams from other liquefiers.

A remaining portion 215 of LNG stream 212, after encountering the split, is directed through expansion device 220 and is then received by passage 108 in reflux condenser 26. The resulting warmed stream 222 travels through cooling passage 224 in the heat exchanger 12 for cooling. The resulting cooled LNG stream 226 is directed through expansion device 228 and is then combined with cooled LNG stream 216 and/or other LNG streams to be collected or for further processing.

Because reflux condenser 26 is cooled with LNG stream 215, a portion of liquid stream 94 is not directed to the reflux condenser as described and shown in FIG. 1. Instead, all of liquid stream 94 is flashed through expansion device 96 and directed to cold temperature separator, such as cold-temperature standpipe 98 to form a cold-temperature liquid stream 100 and a cold-temperature vapor stream 102. The cold-temperature liquid stream 100 and vapor stream 102 are combined (within the heat exchanger 12, within a header of the heat exchanger 12, or prior to entry into a header of the heat exchanger 12) and directed to primary refrigeration passage 104 to provide cooling within the heat exchanger 12.

Turning to FIG. 3, a liquefaction system 310 is illustrated. The system of FIG. 3 operates the same as system 210 disclosed and illustrated in FIG. 2 with the exception that the LNG stream 36 directed to reflux condenser 26 branches off of stream 38 exiting the heat exchanger 12. As shown in FIG. 3, the bottoms liquid stream 36 (which is primarily LNG) exits the NRU column 22 and is cooled in passage 37 in main heat exchanger 12. After the cooled liquid stream 38 exits the heat exchanger 12, it is divided by a split with a portion of the cooled liquid stream traveling through expansion device 312 after exiting the cold box 11 to be collected or further processed. Optionally, after traveling through valve 312, the LNG stream 38 can be combined with LNG stream 322, which was used to cool reflux condenser 26, and/or with other LNG streams from other liquefiers.

A portion of LNG stream 38, after being split, is directed through line 314 and expansion device 316. The stream exiting expansion device 316 is then received by passage 108 in reflux condenser 26 and is used to cool the reflux condenser 26. The resulting warmed stream 318 travels through cooling passage 320 in the heat exchanger 12 for cooling. The resulting cooled LNG stream 322 is directed through expansion device 324 and is then recombined with cooled LNG stream 38 and/or other LNG streams to be collected or for further processing.

Similar to system 210 of FIG. 2, because reflux condenser 26 is cooled with LNG stream 314, a portion of liquid stream 94 is not directed to the reflux condenser 26 as described and shown in FIG. 1. Instead, all of liquid stream 94 is flashed through expansion device 96 and directed to cold temperature separator, such as cold-temperature standpipe 98 to form a cold-temperature liquid stream 100 and a cold-temperature vapor stream 102. The cold-temperature liquid stream 100 and vapor stream 102 are combined (within the heat exchanger 12, within a header of the heat exchanger 12, or prior to entry into a header of the heat exchanger 12) and directed to primary refrigeration passage 104 to provide cooling within the heat exchanger 12.

There are several aspects of the present subject matter which may be embodied separately or together in the methods, devices, and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto.

While the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.