Process for liquefying a natural gas feed stream and removing nitrogen therefrom

Abstract

A method and system for controlling the nitrogen concentration in LNG product and fuel from flash gas within a preferred range. A cooled LNG stream is separated into a nitrogen-rich vapor stream, a fuel stream, and an LNG product stream using multiple phase separation devices such as flash tanks or distillation columns. A portion of the vapor stream is recycled back to the distillation column as reflux. A portion of the stream with a higher nitrogen concentration is combined with the fuel stream to maintain the fuel stream within the desired nitrogen concentration range.

Description

Technical Field

[0001] The present invention relates to a method for use in liquefied natural gas feed streams and for removing nitrogen therefrom. Background Technology

[0002] The fuel used in liquefied natural gas (LNG) liquefaction processes typically comes from flash gas generated at the cold end of the liquefaction unit. Subcooling LNG to prevent flashing in storage tanks requires significant refrigeration power. Allowing some flashing helps reduce refrigeration power. Furthermore, flashing provides a simple method to remove nitrogen from the feed gas to maintain nitrogen levels at or below LNG storage specifications. Flash gas will contain a higher concentration of nitrogen than the feed gas. Therefore, variations in the nitrogen content of the natural gas feed will also affect the nitrogen concentration in the flash gas.

[0003] Since flash gas is used as fuel for gas turbines, it is desirable to limit the nitrogen concentration in the flash gas. In many applications, a maximum nitrogen content of 15-50% is required if flash gas is to be used to fuel certain gas turbines. If the nitrogen content in the flash gas exceeds the maximum nitrogen content for that gas turbine, it may be necessary to release it into the atmosphere and remove the excess nitrogen in its pure components.

[0004] There are many known solutions for removing nitrogen from natural gas streams, including flash tanks, nitrogen stripping towers, integrated nitrogen distillation towers, and stand-alone nitrogen removal units (NRUs). These solutions typically produce compliant LNG and gas turbine fuel streams. In some cases, the fuel stream will contain a relatively high concentration of nitrogen. If the nitrogen concentration is too high (fuel quality too low), additional nitrogen needs to be removed to produce a high-purity nitrogen stream to be discharged and a lean nitrogen fuel stream.

[0005] One existing solution is to use a nitrogen stripping tower and reflux unit to produce compliant LNG products and high-purity nitrogen. This is a satisfactory solution when the LNG plant is driven by electric motors that utilize grid electricity and does not require fuel. However, this solution is relatively inflexible and may result in higher denitrification rates than required to meet LNG product specifications.

[0006] Other existing solutions include expansion processes for multi-component separation, nitrogen towers using LNG for cooling, and NRUs (nitrogen refrigeration units) that produce LNG and emit nitrogen. None of these solutions can control the nitrogen content in the LNG product and fuel stream. Therefore, these solutions typically produce fuel streams with nitrogen content far below the maximum permissible level, leading to unnecessary power consumption, reduced gas turbine power output, and the need for larger and more complex equipment. Therefore, an improved system and method are needed to control the nitrogen concentration in the fuel stream and stored LNG product. Summary of the Invention

[0007] This invention provides a simplified summary of some concepts that will be further described in the detailed description below. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter.

[0008] Embodiments of the present invention provide a comprehensive solution for removing nitrogen from only a portion of the flash gas during LNG liquefaction. A first portion of the flash gas from a nitrogen stripping tower or end flash tank is fed to a distillation column. A nitrogen-rich vapor phase containing only trace amounts of hydrocarbons is drawn from the top of the distillation column. This nitrogen-rich vapor phase is heated in a heat exchanger by high-pressure circulating steam and warm feed gas to produce a heated nitrogen-rich vapor phase. The heated nitrogen-rich vapor phase is separated into a first portion, which will be recompressed to become a recirculation / reflux stream for the distillation column, and a second portion, which is discharged into the atmosphere. The first portion is compressed to form a high-pressure vapor stream, which is circulated, cooled, and liquefied by a cold, low-pressure nitrogen-rich vapor stream and a cold liquid stream from the bottom of the distillation column. The liquefied high-pressure recirculation stream is then returned to the distillation column. Before being heated by the high-pressure circulating steam stream and the warm feed gas stream in the aforementioned heat exchanger, the liquid from the bottom of the distillation column is combined with a second portion of flash gas (flash gas bypass stream) from the nitrogen stripping column or the end flash tank. This heated combined stream is then sent to the end flash compressor as fuel for the gas turbine drive.

[0009] Adjusting the flow rate of the circulating high-pressure stream and the bypass of the flash gas in the distillation column allows for strict composition control of the nitrogen and fuel gas streams emitted into the atmosphere.

[0010] Alternatively, the liquid from the tower can be pumped to reduce the required end flash compressor power and provide a tighter match for the cooling profile in the exchanger.

[0011] The following outlines several aspects of the system and methodology.

[0012] Aspect 1: A method for using and removing nitrogen from a liquefied natural gas feed stream, the method comprising: (a) Cooling and at least partially liquefying the natural gas feed stream to form a cooled LNG stream with a cooled LNG nitrogen concentration; (b) Multiple phase separations are performed in downstream fluid communication with the cooled LNG stream to produce a nitrogen vapor stream having a nitrogen concentration of vapor stream, a fuel stream having a nitrogen concentration of fuel stream, and an LNG product stream having a nitrogen concentration of LNG product stream, wherein the nitrogen concentration of vapor stream is greater than the nitrogen concentration of the cooled LNG, the nitrogen concentration of the fuel stream, and the nitrogen concentration of the LNG product stream. (c) Combining the fuel stream with a mixed stream to produce a fuel product stream having a nitrogen concentration greater than that of the fuel stream, the mixed stream being in downstream fluid flow communication with the cooled LNG stream; and (d) The circulation includes a portion of the nitrogen vapor stream as reflux to the distillation column; Thus, step (c) enables the nitrogen concentration of the fuel product stream to be controlled independently of the nitrogen concentration of the LNG product stream.

[0013] Aspect 2: According to the method of aspect 1, step (c) further includes controlling the flow of the mixed stream to produce a fuel product stream nitrogen concentration within a predetermined fuel product stream nitrogen concentration range.

[0014] Aspect 3: The method according to any one of Aspects 1 to 2, wherein at least one of the plurality of phase separations in step (b) is carried out in a distillation column, and the fuel stream is drawn as a liquid from the bottom of the distillation column.

[0015] Aspect 4: The method according to any one of aspects 1 to 3 further comprises: (e) Cool the circulating stream with the nitrogen vapor stream and the fuel stream.

[0016] Aspect 5: According to the method of aspect 4, step (e) further includes evaporating the fuel stream to cool the circulating stream.

[0017] Aspect 6: The method according to any one of aspects 1 to 5 further comprises: (d1) The circulating flow is compressed and subjected to ambient heat exchange before step (e).

[0018] Aspect 7: The method according to any one of aspects 1 to 6 further comprises: (f) Using the fuel product stream to power at least one gas turbine.

[0019] Aspect 8: The method according to aspect 7 further includes: (g) Using the fuel product stream to drive at least one compressor, the compressor being adapted to compress the refrigerant used in step (a).

[0020] Aspect 9: The method according to any one of Aspects 1 to 8, wherein the plurality of phase separations further comprises phase separating the cooled LNG stream to produce a flash stream and the LNG product stream.

[0021] Aspect 10: The method according to aspect 9, wherein a flash tank is used to separate the cooled LNG stream.

[0022] Aspect 11: The method according to any one of Aspects 1 to 10, wherein a distillation column is used to separate the cooled LNG stream.

[0023] Aspect 12: The method according to any one of Aspects 1 to 11, wherein the plurality of phase separation further comprises introducing at least a first portion of the flash vapor stream into the distillation column to produce the nitrogen vapor stream and the fuel stream.

[0024] Aspect 13: According to the method of aspect 12, the mixed stream includes a second portion of the flash vapor stream.

[0025] Aspect 14: The method according to any one of aspects 1 to 13, wherein the mixed stream is combined with the fuel downstream of step (e). Aspect 15: The method according to any one of aspects 1 to 14, wherein the mixed stream is compressed and cooled before being combined with the fuel stream.

[0026] Aspect 16: The method according to aspects 1 to 15 further includes: (h) Discharge the nitrogen vapor stream.

[0027] Aspect 17: The method according to any one of aspects 1 to 16 further comprises: (h) Store the nitrogen vapor stream.

[0028] Aspect 18: A method for using and removing nitrogen from a liquefied natural gas feed stream, the method comprising: (a) The liquefied natural gas feed stream is at least partially liquefied in the main cryogenic heat exchanger to form a cooled LNG stream with a cooled LNG nitrogen concentration; (b) Separating the cooled LNG stream into an LNG product stream having the nitrogen concentration of the LNG product stream and a flash vapor stream having the nitrogen concentration of the flash vapor stream in a flash tank or distillation column; (c) Separating at least a first portion of the flash stream in a distillation column to produce a nitrogen vapor stream having a vapor stream nitrogen concentration and a fuel stream having a fuel stream nitrogen concentration; (d) Combining the fuel stream with a mixed stream to form a fuel product stream having a higher nitrogen concentration than the fuel stream, wherein the mixed stream includes a second portion of the flash stream; (e) Divide the nitrogen vapor stream into a circulating stream and an exhaust stream; (f) Compressing and cooling the circulating flow; and (g) The circulating stream is further cooled by indirect heat exchange with the fuel stream and the nitrogen vapor stream.

[0029] Aspect 19: According to the method of aspect 19, wherein the nitrogen concentration of the vapor stream is greater than the nitrogen concentration of the cooled LNG, the nitrogen concentration of the fuel stream, and the nitrogen concentration of the LNG product stream.

[0030] Aspect 20: The method according to aspects 19 to 19, wherein step (d) enables the nitrogen concentration of the fuel product stream to be controlled independently of the nitrogen concentration of the LNG product stream.

[0031] Aspect 21: The method according to any one of aspects 18 to 20, wherein step (d) further includes controlling the flow of the mixed stream to produce a fuel product stream nitrogen concentration within a predetermined range of fuel product stream nitrogen concentrations. Attached Figure Description

[0032] The present invention will be described in conjunction with the accompanying drawings, wherein the same numbers denote the same elements.

[0033] Figure 1 This is a schematic flow diagram depicting a method and apparatus for liquefying and removing nitrogen from a natural gas stream according to a first embodiment of the present invention.

[0034] Figure 2 This is a schematic flowchart depicting a method and apparatus according to a second embodiment of the present invention.

[0035] Figure 3 This is a schematic flowchart describing the method and apparatus according to a third embodiment of the present invention.

[0036] Figure 4 This is a schematic flowchart describing the method and apparatus according to the fourth embodiment of the present invention.

[0037] Figure 5 This is a schematic flowchart describing the method and apparatus according to the fifth embodiment of the present invention.

[0038] Figure 6 This is a schematic flowchart describing the method and apparatus according to the sixth embodiment of the present invention.

[0039] Figure 7 This is a schematic flowchart depicting a method and apparatus according to a seventh embodiment of the present invention.

[0040] Figure 8 This is a schematic flowchart describing the method and apparatus according to the eighth embodiment of the present invention.

[0041] Figure 9 This is a schematic flowchart describing the method and apparatus according to the ninth embodiment of the present invention. Detailed Implementation

[0042] The following detailed description provides only preferred exemplary embodiments and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the subsequent detailed description of preferred exemplary embodiments will provide those skilled in the art with an effective description for implementing the preferred exemplary embodiments of the invention. Various changes can be made to the function and arrangement of the elements without departing from the spirit and scope of the invention.

[0043] To aid in the description of the invention, directional terms (e.g., up, down, left, right, etc.) may be used in the specification and claims to describe parts of the invention. These directional terms are intended only to aid in the description and claim of the invention and are not intended to limit the invention in any way. Furthermore, the figures introduced in conjunction with the drawings in the specification may be repeated in one or more subsequent drawings without requiring additional description in the specification to provide context for other features.

[0044] Unless otherwise stated, the articles “a” and “an” as used herein, when applied to any feature in the embodiments of the invention described in the specification and claims, mean one or more. The use of “a” and “an” does not limit the meaning to a single feature unless such limitation is specifically stated. The article “described” preceding a singular or plural noun or noun phrase indicates a particular or specific feature and may have a singular or plural meaning depending on the context in which it is used.

[0045] As used in the specification and claims, the term "conduit" refers to one or more structures through which fluid can be transported between two or more components of a system. For example, a conduit may include pipes, conduits, channels, and combinations thereof for conveying liquids, vapors, and / or gases.

[0046] As used in the specification and claims, the term "flow connectivity" or "fluid flow connectivity" is intended to mean a connection (direct or indirect) between two or more elements to enable fluid to flow between the elements, including connections that may include valves, gates, tees or other devices that may selectively restrict, combine or separate fluid flow.

[0047] As used in the specification and claims, the term "natural gas" refers to a mixture of hydrocarbon gases consisting primarily of methane. As used herein, the term "natural gas" also encompasses synthetic and alternative natural gas. The natural gas feed stream comprises methane and nitrogen (methane is typically the predominant component). Typically, the natural gas feed stream has a nitrogen concentration of 1 to 10 mol%, and the methods and apparatus described herein can efficiently remove nitrogen from the natural gas feed stream even when the nitrogen concentration in the natural gas feed stream is relatively low, such as 5 mol% or less.

[0048] Natural gas streams typically also contain other components, such as one or more other hydrocarbons and / or other components such as helium, carbon dioxide, hydrogen, etc. However, it should not contain any other components that would freeze at concentrations during the cooling and liquefaction of the stream and in the main heat exchanger. Therefore, if necessary, the natural gas feed stream can be pretreated before being introduced into the main heat exchanger to remove water, acid gases, mercury, and heavy hydrocarbons, thereby reducing any such components in the natural gas feed stream to concentration levels that would not cause any freezing problems.

[0049] As used in the specification and claims, the terms “hydrocarbon,” “hydrocarbon gas,” or “hydrocarbon fluid” refer to a gas / fluid comprising at least one hydrocarbon and comprising at least 80 mol% of the hydrocarbon, and more preferably at least 90 mol% of the total gas / fluid composition.

[0050] Unless otherwise stated herein, any and all percentages determined in the specification, drawings, and claims shall be understood to be on a mole % basis. Unless otherwise stated herein, any and all pressures identified in the specification, drawings, and claims shall be understood to be average gauge pressure.

[0051] As used in the specification and claims, the term "compression system" is defined as one or more compression stages. For example, a compression system may include multiple compression stages within a single compressor. In alternative instances, a compression system may include multiple compressors connected in parallel or in series.

[0052] Unless otherwise stated herein, introducing flow at a location means introducing substantially all flow at that location. All flows discussed in the specification and shown in the accompanying drawings (generally indicated by lines with arrows showing the general direction of fluid flow during normal operation) should be understood as being contained within the corresponding conduit. Each conduit should be understood to have at least one inlet and at least one outlet. Furthermore, each piece of equipment should be understood to have at least one inlet and at least one outlet.

[0053] In the claims, letters are used to identify the claimed steps (e.g., (a), (b), and (c)). These letters are used to help refer 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 an order is specifically recited in the claims.

[0054] It should be noted that even though the exemplary natural gas liquefaction system embodiments disclosed herein all have closed-loop refrigeration, the inventive concepts disclosed herein are equally applicable to natural gas liquefaction systems using open-loop or closed-loop compression.

[0055] As used herein, and unless otherwise stated, a stream is “nitrogen-rich” if the nitrogen concentration in the stream is higher than that in the natural gas feed stream. A stream is “nitrogen-lean” if the nitrogen concentration in the stream is lower than that in the natural gas feed stream.

[0056] In the methods and apparatus described herein, unless otherwise stated, the flow, whether liquid or two-phase, can expand and / or partially evaporate by passing the flow through any suitable expansion device. For example, it can expand and partially evaporate by passing the flow through an expansion valve or JT valve or any other device used to influence the flow with (substantially) isenthalpic expansion (and thus flash evaporation). Alternatively or alternatively, it can expand and partially evaporate, for example, by passing the flow through a work-extracting device (such as, for example, a hydraulic device) and expanding the work.

[0057] As used herein, the term “phase separation” is intended to include any process that separates one or more input streams into gaseous and liquid output streams, such as distillation, rectification, stripping, and simple flash separation.

[0058] As used herein, the term "distillation column" is intended to refer to a column that performs only distillation, as well as a column that incorporates both distillation and stripping.

[0059] As used herein, the term "main heat exchanger" refers to a heat exchanger responsible for cooling all or part of the liquefied natural gas stream to produce a cooled LNG stream. A heat exchanger may consist of one or more cooling sections arranged in series and / or parallel. Each such section may constitute a heat exchanger unit with its own separate housing, but similarly, sections may be combined into a single heat exchanger unit sharing a common housing. A heat exchanger unit may be of any suitable type, such as, but not limited to, shell-and-tube, wound coil, or plate-fin heat exchanger units. In such units, each cooling section typically comprises its own bundle of tubes (where the unit is shell-and-tube or wound coil type) or bundle of plate-fins (where the unit is plate-fin type).

[0060] Part or all of the cooling in the main heat exchanger is provided by a closed-loop refrigeration system, in which the circulating refrigerant passes through and is heated in the main heat exchanger, and then passes through and is heated in the condenser heat exchanger. The closed-loop refrigeration system can be of any suitable type. Exemplary refrigeration systems comprising one or more closed-loop systems that can be used according to the invention include single-mixed refrigerant (SMR) systems, dual-mixed refrigerant (DMR) systems, mixed propane-mixed refrigerant (C3MR) systems, AP-X® systems (C3MR or DMR with nitrogen expansion cycle subcooling), vapor expansion cycles using nitrogen, methane, other gases, or mixtures, and other multi-stage cascade refrigeration systems, including the ConocoPhillips Optimized Cascade® process.

[0061] refer to Figure 1 This illustration shows an apparatus and method for removing nitrogen from a natural gas feed stream in a natural gas liquefaction system 100 according to an embodiment of the invention. The natural gas feed stream 101 is introduced into the hot end of a main cryogenic heat exchanger (MCHE) 102. A cooled LNG stream 111 is drawn from the cold end of the MCHE 102. Figure 1 In the depicted embodiment, the MCHE 102 comprises two cooling sections connected in series: a hot section 103 in which the natural gas feed stream 101 is pre-cooled, and a cold section 105 in which the natural gas feed stream 101 is quenched and subcooled. The end of the hot section 103 from which the natural gas feed stream 101 is introduced thus constitutes the hot end of the MCHE 102, and the end of the cold section 105 from which the cooled LNG stream 111 is extracted thus constitutes the cold end of the MCHE 102.

[0062] As will be recognized, the terms “hot” and “cold” in this document refer only to the relative temperature within the cooling section and do not imply any particular temperature range. Each of these sections can constitute a heat exchanger unit with its own separate shell, sleeve, or other form of enclosure, but all parts can be combined into a single heat exchanger unit sharing a common enclosure. The heat exchanger can be any suitable type, such as, but not limited to, shell-and-tube, wound coil, or plate-fin heat exchanger units. In such a unit, each cooling section typically comprises its own bundle of tubes (where the unit is shell-and-tube or wound coil type) or bundle of plate-fins (where the unit is plate-fin type).

[0063] exist Figure 1In the embodiment depicted, cooling for MCHE 102 is provided by a heat-jet refrigerant 104, which is depressurized to form a first cold refrigerant stream 106. This first cold refrigerant stream passes through the housing side of the hot section 103, where it provides cooling to the hot section. A cold-jet refrigerant 108 is depressurized to form a second cold-jet refrigerant stream 110, which passes through the housing side of the cold section 104 to provide cooling to the cold section. The first and second refrigerants can be the same or different refrigerants, and can be part of the same or different open-loop or closed-loop refrigeration systems.

[0064] Cooled LNG stream 111 is drawn from the cold end of MCHE 102 and expanded through JT valve 112 to produce expanded cooled LNG stream 113, which is then depressurized in flash tank 114. Liquid LNG product stream 115 from the bottom of flash tank 114 is pumped by pump 126 and delivered to LNG storage tank (not shown) via conduit 128. Preferably, LNG stream 115 has a nitrogen content of less than 1 mol%.

[0065] Flash stream 116 is drawn from the top of flash tank 114, and a first portion 123 of flash stream 116 is sent to distillation column 130. Flash stream 116 is nitrogen-rich relative to the cooled LNG stream 111. Preferably, flash stream 116 has a nitrogen content between 10 and 50 mol%. In this example, the nitrogen content of flash stream 116 is between 30 and 35 mol%. A second portion 117 of flash stream 116 forms bypass stream 117, which will be discussed in more detail herein.

[0066] The overhead stream 131 from distillation column 130 is heated in the terminal flash exchanger 121 to produce a nitrogen vapor stream 132 suitable for discharge into the atmosphere 135 via valve 134. The nitrogen vapor stream 132 preferably has a nitrogen concentration greater than 90 mol%, and more preferably greater than 99 mol%. The nitrogen vapor stream 132 preferably has a methane molar concentration not exceeding 1000 ppm, and more preferably in the range of 0.1 ppm to 1000 ppm.

[0067] The liquid stream from the distillation column, also known as fuel stream 119, is drawn from the bottom of the distillation column 130 and optionally combined with a second portion 117 of the flash stream 116 to form a fuel product stream 120. This product stream is then heated and evaporated in the terminal flash exchanger 121 to produce a vaporized fuel stream 122. The vaporized fuel stream 122 is nitrogen-lean relative to the cooled LNG stream 111 and preferably has a nitrogen concentration very close to, but not exceeding, the maximum fuel gas concentration required by the equipment being fueled.

[0068] A second portion 117 of the flash vapor stream 116 is directed to the liquid stream 119 of the distillation column upstream of the distillation column 130, and a component (in this example, valve 118) is provided to control the flow rate of the second portion 117, enabling the nitrogen content of the vaporized fuel stream 122 to be controlled independently of the nitrogen content of the flash vapor stream 116. The second portion 117 of the flash vapor stream 116 serves as a mixing stream, which can be used to control the nitrogen content of the vaporized fuel stream 122. This allows the use of a smaller capacity distillation column 130, reducing the power required for distillation, and maximizing the power output of the gas turbine when the nitrogen content of the flash vapor stream 116 is higher than the maximum allowable value in the fuel. For example, if the maximum acceptable nitrogen concentration of the fuel stream 122 is 15 mol%, the flow rate of the second portion 117 of the flash vapor stream 116 can be controlled to maintain the nitrogen concentration of the fuel stream 122 between 13 and 15 mol%, preferably within 5%, 2%, or 1% of the maximum nitrogen concentration in the gas turbine fuel.

[0069] Preferably, only a first portion 133 of the nitrogen vapor stream 132 is discharged into the atmosphere 135 through valve 134. A second portion 136 of the nitrogen vapor stream 132 is recompressed in compressor 137 to produce a compressed nitrogen stream 138. The compressed nitrogen stream 138 is cooled in cooler 139 to produce a cooled, compressed nitrogen stream 140. The cooled, compressed nitrogen stream 140 is further cooled and liquefied in terminal flash exchanger 121 to produce a liquefied nitrogen stream 141. The liquefied nitrogen stream 141 expands in JT valve 142 to produce a reflux stream 143, which returns to the top of distillation column 130 to provide reflux to distillation column 130. If the nitrogen concentration in the cooled LNG stream 111 drops below a predetermined value, valve 134 can be closed if / when the nitrogen concentration in the cooled LNG stream 111 increases, and then reopened. In other words, when denitrification is not required, the denitrification subsystem can be "idled" and ready to react quickly if / when nitrogen concentration increases.

[0070] Alternatively, the mixed stream may comprise a portion of stream 133 instead of a second portion 117 of flash stream 116. This would allow the mixed stream to be drawn downstream of the end flash exchanger 121, making the mixed stream single-phase (instead of the potential two-phase second portion 117).

[0071] Figure 2 Another exemplary embodiment of a natural gas liquefaction system 200 is shown. The components of this embodiment are also referenced. Figure 1 Shown and described in Figure 2 The reference identifier number has increased by 100. For example, Figure 1 MCHE 102 in the text corresponds to Figure 2 MCHE 202 in the middle. Figure 2 The markings in the middle and Figure 1The corresponding components in the [context] are indistinguishable and may not be combined. Figure 2 This will be discussed in the instruction manual.

[0072] exist Figure 2 In the embodiment shown, Figure 1 The flash tank 114 shown is replaced by a reboiler 205 and a nitrogen stripping column 224 to increase the nitrogen concentration in the nitrogen-rich gas stream (flash gas) 216 fed into the distillation column 230. The cooled LNG stream 211 from the MCHE is subcooled in the reboiler 205 with the bottom stream 209 from the nitrogen stripping column 224. The bottom liquid stream 215 from the nitrogen stripping column 215 is then pumped by a pump 226 and a conduit 228 to a storage tank.

[0073] Figure 3 Another exemplary embodiment of the natural gas liquefaction system 300 is shown, wherein a distillation column 330 is disposed upstream of a flash tank 314. See also... Figure 1 The elements of the embodiment shown and described are in Figure 3 The reference identifier number has increased by 200. For example, Figure 1 MCHE 102 in the text corresponds to Figure 3 MCHE 302 in the middle. Figure 3 The markings in the middle and Figure 1 The corresponding components in the [context] are indistinguishable and may not be combined. Figure 3 Discussed in the specification. Cooled LNG stream 311 drawn from MCHE 302 is depressurized via valve 312 to form expanded, cooled LNG stream 313 entering distillation column 330. The overhead stream 331 of the nitrogen-rich distillation column is heated in NRU exchanger 321 to form nitrogen vapor stream 332. A first portion of nitrogen vapor stream 333 is vented to atmosphere 335. A second portion 336 of nitrogen vapor stream 332 is recompressed in compressor 337, cooled in cooler 339, and liquefied in NRU exchanger 321 for use as reflux in distillation column 330. In this example, the operating pressure of distillation column 330 is higher than that of flash tank 314. Changing the pressure differential between distillation column 330 and flash tank 314 allows control of streams 355 and 306, and thus ultimately provides independent control of the nitrogen concentration of this fuel product stream 361.

[0074] Liquid stream 319 is drawn from the bottom of distillation column 330. A first portion 379 of liquid stream 319 is pumped by pump 305 to become stream 306 and evaporated in NRU exchanger 321 for final use as fuel 361. A second portion 343 of liquid stream 319 is expanded in JT valve 312 and flashed in flash tank 314. Vapor stream 355 from the top of flash tank 314 is heated by natural gas feed stream 350 in terminal flash exchanger 351 to form heated vapor stream 356 and LNG stream 352. LNG stream 352 expands as it passes through expansion valve 353 to form expanded LNG stream 354, which is introduced into flash tank 314. Heated vapor stream 356 is compressed in compressor 357 to form compressed heated vapor stream 358. The compressed, heated vapor stream 358 is cooled in a cooler 359 to form a cooled, compressed vapor stream 360. The cooled, compressed vapor stream 360 is combined with vaporized liquid 324 from a distillation column 330 to form a combined fuel product stream 361. Similar to... Figure 1 The nitrogen concentration of fuel product stream 361 is slightly lower than that of fuel suitable for use in gas turbines. For example, if the maximum acceptable nitrogen content of fuel gas in a gas turbine is 30 mol%, the nitrogen content of fuel product stream 361 is preferably between 28 and 30 mol%. Stream 350 is cooled by the overhead vapor stream in NRU exchanger 321 and sent to flash tank 314.

[0075] A second portion 343 of the distillation column liquid stream 319 is directed to a flash tank 314 to generate a top vapor stream 355. A component (in this example, valve 312) is provided to control the flow rate of the second portion 343, enabling the nitrogen concentration of the vaporized fuel stream 361 to be controlled independently of the nitrogen content of the first portion 379 of the distillation column liquid stream 319. The second portion 343 serves as a mixing stream, which can be used to control the nitrogen content of the vaporized fuel stream 361. This allows for the use of a lower-capacity distillation column 330 and reduces the power required for distillation. For example, if the maximum acceptable nitrogen concentration of the vaporized fuel stream 361 is 15 mol%, the flow rate of the second portion 343 of the distillation column liquid stream can be controlled to maintain the nitrogen concentration of the fuel stream 361 between 10 and 15 mol%.

[0076] Figure 4 Another alternative embodiment of the natural gas liquefaction system 400 is shown. See also... Figure 1 The elements of the embodiment shown and described are in Figure 4 The reference identifier number has increased by 300. For example, Figure 1 MCHE 102 in the text corresponds to Figure 4 MCHE 402 in the middle. Figure 4 The markings in the middle and Figure 1 The corresponding components in the [context] are indistinguishable and may not be combined. Figure 4 This will be discussed in the instruction manual. Figure 4 In this embodiment, a high-pressure flash tank 414 is used to improve the efficiency of the nitrogen distillation column 430. Cooled LNG stream 411 from MCHE 402 is expanded via JT valve 412 and depressurized in the high-pressure flash tank 414. Flash stream 416 is drawn from the top of the high-pressure flash tank 414 and fed into the distillation column 430 to further remove nitrogen from the flash and fuel gas. Overhead stream 431 is drawn from the top of the distillation column 430 and heated in the NRU / terminal flash exchanger 421 to form a nitrogen stream 432. A first portion of the nitrogen stream 433 is discharged into the atmosphere 435. A second portion of the nitrogen stream 436 is recompressed in compressor 437, cooled in cooler 439, and liquefied in the NRU / terminal flash exchanger 421 as reflux 443 for the nitrogen distillation column 430.

[0077] Liquid stream 419 from the bottom of distillation column 430 is pumped to NRU / terminal flash exchanger 421 for final use as fuel 461.

[0078] Liquid LNG stream 415 is drawn from the bottom of high-pressure flash tank 414. The liquid LNG stream 415 from high-pressure flash tank 414 expands through JT valve 462, and then flashes and separates in LNG flash tank 463. Top vapor stream 465 is drawn from the top of LNG flash tank 463. Top vapor stream 465 is heated in an NRU / end flash exchanger to form heated vapor stream 456. Heated vapor stream 456 is compressed in compressor 457 to form compressed heated vapor stream 455. Compressed heated vapor stream 455 is cooled in cooler 459 to form cooled compressed vapor stream 460. Cooled compressed vapor stream 460 is combined with vaporized liquid from distillation column 424 to form combined fuel stream 461. Preferably, combined fuel stream 461 has a nitrogen concentration of less than 15 mol% and is suitable for use as fuel in a gas turbine.

[0079] Liquid LNG stream 464 is drawn from the bottom of LNG flash tank 463 and pumped to storage tank 428 via pump 426.

[0080] Figure 5 Another alternative embodiment of the natural gas liquefaction system 500 is shown. See also... Figure 1 The elements of the embodiment shown and described are in Figure 5 The number of reference digits in the Chinese database has increased by 400. Figure 1 The 102 in the middle corresponds to the same as Figure 5 It is the same as MCHE 502. Figure 5 The markings in the middle and Figure 1 Components that are indistinguishable from each other may not be combined. Figure 5 This will be discussed in the instruction manual. Figure 5 In one embodiment, a high-pressure flash tank 514 is used to generate a flash vapor stream 516, which is heated and then compressed to fuel pressure to provide a mixed stream 560. The mixed stream 560 is mixed with a fuel stream 524 to control the nitrogen concentration in the combined fuel stream 561.

[0081] refer to Figure 5 The cooled LNG stream 511 from MCHE 502 is depressurized in the high-pressure flash tank 514. Liquid LNG stream 515 is drawn from the bottom of the high-pressure flash tank 514. The liquid LNG stream 515 from the high-pressure flash tank 514 is expanded through the JT valve 562 fed into the distillation column 530 to further remove nitrogen from the LNG.

[0082] A top vapor stream 531 is drawn from the top of distillation column 530. Compared to the liquid LNG stream 515, the top vapor stream 531 is nitrogen-rich. The top vapor stream 531 is heated in the NRU / terminal flash exchanger 521 to form a nitrogen stream 532. A first portion of the nitrogen stream 533 is discharged into the atmosphere 535, and a second portion of the nitrogen stream 536 is recompressed in compressor 537, cooled in cooler 539, and liquefied in the NRU / terminal flash exchanger 521 as reflux to column 543.

[0083] A first portion 579 of the distillation column liquid stream 563 is drawn from the bottom of the distillation column 530 and pumped by pump 505 to the NRU / terminal flash exchanger 521, where the first portion of the distillation column liquid stream is heated and evaporated to form a gas stream, which is ultimately used as fuel 561. A second portion 564 of the distillation column liquid stream 563 is transferred to the storage tank 528 by pump 526.

[0084] Flash vapor stream 516 from high-pressure flash tank 514 is heated in NRU / terminal flash exchanger 521 to form heated vapor stream 556. Heated vapor stream 556 is compressed in compressor 557 to form compressed heated vapor stream 558. Compressed heated vapor stream 558 is cooled in cooler 559 to form cooled compressed vapor stream 560. Cooled compressed vapor stream 560 is mixed with vaporized liquid from distillation column 524 to form combined fuel stream 561. Preferably, the nitrogen concentration of combined fuel stream 561 is below the maximum nitrogen concentration in the fuel (e.g., 13-15 mol% if the maximum nitrogen concentration in the fuel is 15%) and is suitable for use as fuel in a gas turbine.

[0085] Figure 6 Another alternative embodiment of the natural gas liquefaction system 600 is shown. See also... Figure 1 The elements of the embodiment shown and described are in Figure 6 The number of reference digits in the Chinese database has increased by 500. Figure 1MCHE 102 in the text corresponds to Figure 6 MCHE602 in the middle. Figure 6 The markings in the middle and Figure 1 Components that are indistinguishable from each other may not be combined. Figure 6 This will be discussed in the instruction manual. Figure 6 In one embodiment, a high-pressure flash tank 614 is used to improve the efficiency of the nitrogen distillation column 630.

[0086] refer to Figure 6 The cooled LNG stream 611 from MCHE 602 is depressurized in the high-pressure flash tank 614.

[0087] Flash vapor stream 616 is drawn from the top of high-pressure flash tank 614. Flash vapor stream 616 is cooled in heat exchanger 666 by flash vapor stream 665 from LNG flash tank 663 to form cooled nitrogen-rich vapor stream 668. Cooled nitrogen-rich vapor stream 668 is expanded through JT valve 669 and fed to distillation column 630.

[0088] The overhead vapor stream 631 is drawn from the top of distillation column 630. The overhead vapor stream 631 is heated in the NRU / terminal flash exchanger 621 to form a nitrogen stream 632. A first portion of the nitrogen stream 633 is discharged to the atmosphere 635. A second portion of the nitrogen stream 636 is recompressed in compressor 637, cooled in cooler 639, and liquefied in the NRU / terminal flash exchanger 621 to serve as reflux for distillation column 643.

[0089] The liquid stream 619 of the distillation column is drawn from the bottom of the distillation column 630 and transferred by a pump to the NRU / terminal flash exchanger 621, where the liquid stream of the distillation column forms a vapor stream 624, which is ultimately used as fuel 661.

[0090] A liquid LNG stream 615 is drawn from the bottom of a high-pressure flash tank 614. The liquid LNG stream 615 expands through a JT valve 662 and then flashes and separates in an LNG flash tank 663. The vapor stream 665 from the LNG flash tank 663 is first heated in a heat exchanger 666 by the vapor fed to the distillation column 616, and then further heated in an NRU / terminal flash exchanger 621 to form a heated vapor stream 656. The heated vapor stream 656 is compressed 657 in a compressor to form a compressed heated vapor stream 658. The compressed heated vapor stream 658 is cooled in a cooler 659 to form a cooled compressed vapor stream 660. The cooled compressed vapor stream 660 is combined with vaporized liquid from the distillation column 630 to form a combined fuel stream 661. Preferably, the combined fuel stream 661 has a nitrogen concentration of less than 15 mol% and is suitable for use as fuel in a gas turbine.

[0091] Liquid LNG stream 664 is removed from the bottom of LNG flash tank 663 and transferred to storage tank 628 via pump 626.

[0092] Figure 7 An alternative embodiment of the natural gas liquefaction system 700 is shown. See also... Figure 1 The elements of the embodiment shown and described are in Figure 7 The number of reference digits has increased by 600. For example, Figure 1 MCHE 102 in the text corresponds to Figure 7 MCHE 702 in the middle. Figure 7 The markings in the middle and Figure 1 Components that are indistinguishable in their corresponding parts may not be combined. Figure 7 This will be discussed in the instruction manual. Figure 7 In the embodiments, the high-pressure nitrogen stripping tower 705 is replaced Figure 4 High-pressure flash tank 414.

[0093] refer to Figure 7 The cooled LNG stream 711 from MCHE 702 is subcooled by the reboiler stream 709 from the bottom of the nitrogen stripping column 714. The overhead vapor stream 716 from the high-pressure nitrogen stripping column 714 is fed to the rectification column 730 to remove nitrogen from the flash / fuel gas. The nitrogen vapor from column 731 is heated in the NRU / end flash exchanger 721 to form a nitrogen stream 732. The first portion of the nitrogen stream 733 is discharged to the atmosphere 735. The second portion of the nitrogen stream 736 is recompressed (in compressor 737), cooled (in heat exchanger 739), and liquefied in the NRU / end flash exchanger 721 as reflux 743 for column 730.

[0094] Liquid 719 from distillation column 730 is pumped 705 and evaporated in NRU / end flash exchanger 721 for use as fuel 761. Liquid stream 715 from nitrogen stripping column 714 is flashed and separated in LNG flash tank 763. Vapor stream 765 from LNG flash tank 763 is used to generate a mixed stream for controlling the nitrogen concentration in fuel product stream 761. Vapor stream 765 from LNG flash tank 763 is heated in NRU / end flash exchanger 721 and mixed with vaporized liquid from distillation column 730. The combined fuel stream 761 has a reduced nitrogen content (less than 15% nitrogen), suitable for use as fuel in gas turbines. LNG product stream 725 is drawn from LNG flash tank 763 and sent to storage tank 728.

[0095] Figure 8 Another alternative embodiment of the natural gas liquefaction system 800 is shown. (Refer to...) Figure 1 The elements of the embodiment shown and described are in Figure 8 The number of reference digits has increased by 700. For example, Figure 1 MCHE 102 in the text corresponds to Figure 8 MCHE802 in the middle. Figure 8 The markings in the middle and Figure 1 Components that are indistinguishable from each other may not be combined. Figure 8 This will be discussed in the instruction manual. Figure 8 In the embodiments, Figure 4 The system was modified to allow the NRU / terminal flash exchanger 821 to compress the recycle nitrogen 836 for reflux 843 at a lower temperature than the nitrogen emitted into the atmosphere 835.

[0096] Figure 9 Another alternative embodiment of the natural gas liquefaction system 900 is shown. See also... Figure 1 The elements of the embodiment shown and described are in Figure 8 The number of reference digits has increased by 800. For example, Figure 1 MCHE 102 in the text corresponds to Figure 9 MCHE 902 in the middle. Figure 9 Components marked in the diagram that are identical to their counterparts described in the previous diagrams may not be assembled. Figure 9 This will be discussed in the instruction manual. Figure 9 Implementation examples and Figure 2 The illustrated embodiments are very similar. In this embodiment 900, the mixed stream used for independently controlling the nitrogen content of the fuel gas stream 993 is stream 995, separated from the compressed nitrogen stream 940. This arrangement provides the ability to pump liquid 919 from the distillation column 930, reducing or eliminating the requirement for a fuel compressor. Optionally, the nitrogen content of the fuel gas stream 993 can be controlled by stream 994, separated from the nitrogen vapor stream 932.

[0097] Example Table 1 is... Figure 2 The illustrated embodiment is an example of an operational model of a natural gas liquefaction and denitrification system. This example is based on a feed gas stream 201 with a nominal nitrogen content of 2.5%. The process is optimized to produce LNG with a nitrogen content of less than 0.65%, fuel with a nitrogen content of less than 23.5%, and nitrogen containing 800 ppm methane emitted into the atmosphere.

[0098] Table 1 .

[0099] Therefore, the present invention has been disclosed according to exemplary embodiments and alternative embodiments thereof. Of course, those skilled in the art can make various changes, modifications, and alterations to the teachings of the present invention without departing from its intended spirit and scope. The present invention is intended to be limited only by the terms of the appended claims.

Claims

1. A method for using and removing nitrogen from a liquefied natural gas feed stream, the method comprising: (a) Cooling and at least partially liquefying the natural gas feed stream to form a cooled LNG stream with a cooled LNG nitrogen concentration; (b) Multiple phase separations are performed in downstream fluid flow communication with the cooled LNG stream to produce a nitrogen vapor stream having a nitrogen concentration of vapor stream, a fuel stream having a nitrogen concentration of fuel stream, and an LNG product stream having a nitrogen concentration of LNG product stream, wherein the nitrogen concentration of vapor stream is greater than the nitrogen concentration of the cooled LNG, the nitrogen concentration of the fuel stream, and the nitrogen concentration of the LNG product stream. (c) Combining the fuel stream with a mixed stream to produce a fuel product stream having a nitrogen concentration greater than that of the fuel stream, the mixed stream being in downstream fluid flow communication with the cooled LNG stream; (d) The circulation includes a portion of the nitrogen vapor stream circulating as reflux to the distillation column; and (e) Cooling the circulating stream with the nitrogen vapor stream and the fuel stream; Thus, step (c) enables the nitrogen concentration of the fuel product stream to be controlled independently of the nitrogen concentration of the LNG product stream.

2. The method of claim 1, wherein step (c) further comprises controlling the flow of the mixed stream to produce a fuel product stream nitrogen concentration within a predetermined fuel product stream nitrogen concentration range.

3. The method of claim 1, wherein at least one of the plurality of phase separations in step (b) is carried out in a distillation column, and the fuel stream is drawn as a liquid from the bottom of the distillation column.

4. The method of claim 1, wherein step (e) further comprises evaporating the fuel stream to cool the circulating stream.

5. The method of claim 1, further comprising: (d1) The circulating flow is compressed and subjected to ambient heat exchange before step (e).

6. The method of claim 1, further comprising: (f) Using the fuel product stream to power at least one gas turbine.

7. The method of claim 6, further comprising: (g) Using the fuel product stream to drive at least one compressor, the compressor being adapted to compress the refrigerant used in step (a).

8. The method of claim 1, wherein the plurality of phase separations further comprises phase separating the cooled LNG stream to generate a flash stream and the LNG product stream.

9. The method of claim 8, wherein a flash tank is used to separate the cooled LNG stream.

10. The method of claim 8, wherein a distillation column is used to separate the cooled LNG stream.

11. The method of claim 8, wherein the plurality of phase separation further comprises introducing at least a first portion of the flash vapor stream into the distillation column to generate the nitrogen vapor stream and the fuel stream.

12. The method of claim 11, wherein the mixed stream comprises a second portion of the flash vapor stream.

13. The method of claim 1, wherein the mixed stream is combined with the fuel stream downstream of step (e).

14. The method of claim 1, wherein the mixed stream is compressed and cooled prior to being combined with the fuel stream.

15. The method of claim 1, further comprising: (h) Discharge the nitrogen vapor stream.

16. The method of claim 1, further comprising: (h) Store the nitrogen vapor stream.

17. A method for using and removing nitrogen from a liquefied natural gas feed stream, the method comprising: (a) The liquefied natural gas feed stream is at least partially liquefied in the main cryogenic heat exchanger to form a cooled LNG stream with a cooled LNG nitrogen concentration; (b) Separating the cooled LNG stream into an LNG product stream having the nitrogen concentration of the LNG product stream and a flash vapor stream having the nitrogen concentration of the flash vapor stream in a flash tank or distillation column; (c) Separating at least a first portion of the flash stream in a distillation column to produce a nitrogen vapor stream having a vapor stream nitrogen concentration and a fuel stream having a fuel stream nitrogen concentration; (d) Combining the fuel stream with a mixed stream to form a fuel product stream having a higher nitrogen concentration than the fuel stream, wherein the mixed stream includes a second portion of the flash stream; (e) Divide the nitrogen vapor stream into a circulating stream and an exhaust stream; (f) Compressing and cooling the circulating flow; and (g) The circulating stream is further cooled by indirect heat exchange with the fuel stream and the nitrogen vapor stream.

18. The method of claim 17, wherein the nitrogen concentration of the vapor stream is greater than the nitrogen concentration of the cooled LNG, the nitrogen concentration of the fuel stream, and the nitrogen concentration of the LNG product stream.

19. The method of claim 17, wherein step (d) enables the nitrogen concentration of the fuel product stream to be controlled independently of the nitrogen concentration of the LNG product stream.

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