Semi-open-loop liquefaction process

Abstract

To provide a semi-open loop liquefaction process.SOLUTION: Described herein are methods and systems for liquefying natural gas by: cooling and liquefying a natural gas feed stream via indirect heat exchange with at least a first cold refrigerant stream so as to form a first liquefied natural gas stream and a warmed gaseous refrigerant stream; flashing and separating the first liquefied natural gas stream so as to form a liquefied natural gas product stream and at least a first flash gas stream; combining and compressing the first flash gas stream and the warmed gaseous refrigerant stream so as to form a compressed refrigerant stream; and expanding at least a first portion of the compressed refrigerant stream so as to form the first cold refrigerant stream; where the natural gas feed stream is kept separate from and is not combined with either the first flash gas stream or the compressed refrigerant stream.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method and system for liquefying natural gas, which cools a natural gas feed stream, liquefies it via indirect heat exchange with a stream of one or more refrigerants, and then flashes and separates the resulting liquefied natural gas (LNG) stream to produce an LNG product.

Background Art

[0002] Various methods and systems for liquefying natural gas are known in the art, many of which are described in the paper “An Evolutionary Approach” by Roberts, Bukowski, and Mitchell, Hydrocarbon Engineering, February 2019. Figure 5 of this paper shows the AP-C1™ liquefaction process developed by Air Products, which is also described in a research paper titled “Innovative Liquefaction Technology for Floating LNG” by Mark J. Roberts, Dr. Oznur Saygi-Arslan, Dr. Fei Chen, and Janet F. Mitchell, and is associated with a presentation from 9:40 am to 10:05 am on April 6, 2017, as part of the Floating LNG: Design and Technology Session at the 2017 Gastech Conference & Exhibition in Tokyo, Japan.

[0003] Air Products developed the AP-C1™ liquefaction process to take advantage of the benefits of methane as the working fluid in a Brayton cooling cycle. In the AP-C1™ process, the natural gas feed stream is liquefied via indirect heat exchange with a methane-based refrigerant circulating in a closed-loop reverse Brayton cooling cycle before being flashed to produce the final LNG product.

[0004] U.S. Patent Application No. 2018 / 0180354A1 describes a method for liquefying a natural gas feed stream using an open-loop cycle. In this method, a compressed refrigerant stream leaving a refrigerant compressor is divided into first and second parts, the first part being combined with the natural gas feed stream before the natural gas feed stream is expanded in an expander and separated into a vapor fraction and a liquid fraction in a separator, the vapor fraction being heated in a first heat exchanger before being sent to the refrigerant compressor. The second part of the refrigerant stream is cooled in a first heat exchanger section before being further divided into third and fourth parts, the third part being further cooled and liquefied in a second heat exchanger to provide an LNG product, the fourth part being expanded in an expander and separated into a vapor fraction and a liquid fraction in a separator, the vapor fraction being heated in a second heat exchanger and further heated in a first heat exchanger before being sent to the refrigerant compressor. [Overview of the project]

[0005] A method and system for liquefying natural gas (also referred herein as a “semi-open-loop” method and system) is disclosed herein, wherein a natural gas feedstream is cooled and liquefied via indirect heat exchange with one or more flows of cryogenic refrigerants, and the resulting LNG stream is then flashed and separated to produce flash gas and LNG products. In the disclosed method and system (also referred herein as a “semi-open-loop” method and system), the flash gas and heated gaseous refrigerants are combined and compressed to form a compressed refrigerant that provides refrigerants, and then expanded to provide one or more flows of cryogenic refrigerants, while the natural gas feedstream remains separated from both the flash gas and the compressed refrigerants.

[0006] Several preferred embodiments of the methods and systems according to the present invention are outlined below.

[0007] Embodiment 1: A method for liquefying natural gas, (a) A step of cooling and liquefying a natural gas supply flow through indirect heat exchange with at least a first low-temperature refrigerant flow to form a first liquefied natural gas flow and a heated gaseous refrigerant flow, (b) A step of flashing and separating the first liquefied natural gas stream to form a liquefied natural gas product stream and at least a first flash gas stream, (c) A step of combining a first flash gas flow and a heated gaseous refrigerant flow, compressing them to form a compressed refrigerant flow, (d) The process includes expanding at least a first portion of the compressed refrigerant flow to form a first low-temperature refrigerant flow, A method wherein the natural gas supply stream remains separated from either the first flash gas stream or the compressed refrigerant stream and is not combined with either the first flash gas stream or the compressed refrigerant stream.

[0008] Embodiment 2: The method according to Embodiment 1, wherein step (a) is performed in one or more coil-wound heat exchanger sections of a main coil-wound heat exchanger unit or set of units.

[0009] Embodiment 3: The method according to Embodiment 1 or 2, wherein step (c) comprises compressing the first flash gas flow in one or more flash gas compression steps before combining the first flash gas flow with the heated gaseous refrigerant flow, and compressing the combined first flash gas flow and heated gaseous refrigerant flow in one or more refrigerant compression steps to form a compressed refrigerant flow.

[0010] Appearance 4: The method is (e) A step of taking a first auxiliary natural gas stream from the natural gas supply stream before the natural gas supply stream is cooled and liquefied in step (a), (f) a step of cooling and liquefying the first auxiliary natural gas flow through indirect heat exchange with the first flash gas flow to form a second liquefied natural gas flow, The first flash gas flow is heated in step (f) before being compressed in step (c) and combined with the heated gaseous refrigerant flow. The method according to any one of embodiments 1 to 3, wherein step (b) includes combining, flashing, and separating a second liquefied natural gas stream and a first liquefied natural gas stream to form a liquefied natural gas product stream and at least a first flash gas stream.

[0011] Embodiment 5: The method according to Embodiment 4, wherein step (f) is performed in one or more coil-wound heat exchanger sections of the first flash gas heat exchanger unit or set of units.

[0012] Embodiment 6: The method according to Embodiment 5, wherein the first flash gas heat exchanger unit comprises an integrated heat exchanger and phase separator having a shell casing that encloses one or more coil-wound heat exchanger sections positioned above the phase separator section, the phase separator section being used in step (b) to separate the first flash gas flow from the first and second liquefied natural gas flows.

[0013] Embodiment 7: Step (f) includes pre-cooling, cooling, and liquefying the first auxiliary natural gas flow through indirect heat exchange with the first flash gas flow to form a second liquefied natural gas flow, The method according to any one of embodiments 4 to 6, further comprising: taking a first side flow of natural gas from the first auxiliary natural gas flow after precooling the first auxiliary natural gas flow and before liquefaction; and introducing the first side flow of natural gas into the natural gas supply flow after precooling the natural gas supply flow and before liquefaction of the natural gas supply flow in step (a).

[0014] Embodiment 8: Step (b) includes flashing and separating the first liquefied natural gas stream to form a liquefied natural gas product stream, and at least a first flash gas stream and a second flash gas stream, Step (c) includes combining and compressing the second flash gas flow, the first flash gas flow, and the heated gaseous refrigerant flow to form a compressed refrigerant flow, The method according to any one of embodiments 1 to 7, wherein the natural gas supply stream also remains separated from and is not combined with the second flash gas stream.

[0015] Embodiment 9: The method according to Embodiment 8, wherein step (c) comprises compressing the second flash gas flow in one or more flash gas compression steps before combining the second flash gas flow with the first flash gas flow; then compressing the combined first and second flash gas flows in one or more further flash gas compression steps before combining the combined first and second flash gas flows with the heated gaseous refrigerant flow; and compressing the combined first and second flash gas flows and the heated gaseous refrigerant flow in one or more refrigerant compression steps to form a compressed refrigerant flow.

[0016] Embodiment 10: The method is (e) A step of taking a first auxiliary natural gas stream and a second auxiliary natural gas stream from the natural gas supply stream before the natural gas supply stream is cooled and liquefied in step (a), (f) A step of forming a second liquefied natural gas flow by cooling and liquefying the first auxiliary natural gas flow through indirect heat exchange with the first flash gas flow, (g) a step of cooling and liquefying the second auxiliary natural gas flow through indirect heat exchange with the second flash gas flow to form a third liquefied natural gas flow, The first flash gas flow is compressed in step (c) and heated in step (f) before being combined with the second flash gas flow and the heated gaseous refrigerant flow. The second flash gas flow is compressed in step (c) and heated in step (g) before being combined with the first flash gas flow and the heated gaseous refrigerant flow. The method according to embodiment 8 or 9, wherein step (b) includes combining, flushing, and separating a second liquefied natural gas stream and a first liquefied natural gas stream to form a fourth liquefied natural gas stream and a first flash gas stream, and then combining, flushing, and separating the fourth liquefied natural gas stream and a third liquefied natural gas stream to form a liquefied natural gas product stream and at least a second flash gas stream.

[0017] Embodiment 11: The method according to Embodiment 10, wherein step (f) is performed in one or more coil-wound heat exchanger sections of a first flash gas heat exchanger unit or set of units, and step (g) is performed in one or more coil-wound heat exchanger sections of a second flash gas heat exchanger unit or set of units.

[0018] Embodiment 12: The first flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing that includes one or more coil-wound heat exchanger sections positioned above the phase separator section, wherein the phase separator section is used in process (b) to separate the first flash gas flow from the first and second liquefied natural gas flows. The method according to embodiment 11, wherein the second flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing that includes one or more coil-wound heat exchanger sections positioned above the phase separator section, the phase separator section being used in step (b) to separate the second flash gas flow from the third and fourth liquefied natural gas flows.

[0019] Embodiment 13: Step (f) includes pre-cooling, cooling, and liquefying the first auxiliary natural gas flow through indirect heat exchange with the first flash gas flow to form a second liquefied natural gas flow, Step (g) includes pre-cooling, cooling, and liquefying the second auxiliary natural gas flow via indirect heat exchange with the second flash gas flow to form a third liquefied natural gas flow, The method further comprises: taking out a first sidestream of natural gas from the first auxiliary natural gas stream after precooling and before liquefaction of the first auxiliary natural gas stream; taking out a second sidestream of natural gas from the second auxiliary natural gas stream after precooling and before liquefaction of the second auxiliary natural gas stream; and introducing the first sidestream of natural gas and the second sidestream of natural gas into the natural gas feed stream after precooling and before liquefaction of the natural gas feed stream in step (a). The method according to any one of aspects 10 to 12.

[0020] Aspect 14: The method comprises (h) introducing the liquefied natural gas product stream into a liquefied natural gas storage tank and storing the liquefied natural gas product in the liquefied natural gas storage tank; (i) taking out a boil-off gas stream from the liquefied natural gas storage tank. The method further comprises step (c) comprises combining, compressing the boil-off gas stream, the first flash gas stream, and the heated gaseous refrigerant stream to form a compressed refrigerant stream, The natural gas feed stream also remains separated from the boil-off gas stream and is not combined with the boil-off gas stream. The method according to any one of aspects 1 to 13.

[0021] Aspect 15: Step (c) comprises compressing the boil-off gas stream in one or more boil-off gas compression stages before combining the boil-off gas stream, the first flash gas stream, and the heated gaseous refrigerant stream, and compressing the combined boil-off gas stream, the first flash gas stream, and the heated gaseous refrigerant stream in one or more refrigerant compression stages to form a compressed refrigerant stream. The method according to aspect 14.

[0022] Aspect 16: Step (d) comprises expanding a first portion of the compressed refrigerant stream to form a first low-temperature refrigerant stream, step (b) comprises combining, flashing, and separating the first liquefied natural gas stream and the second low-temperature refrigerant stream to form a liquefied natural gas product stream and at least a first flash gas stream, The method comprises (j) The method according to any one of embodiments 1 to 15, further comprising the step of cooling a second portion of a compressed refrigerant flow through indirect heat exchange with a first low-temperature refrigerant flow to form a second cooling refrigerant flow.

[0023] Embodiment 17: The method is (k) Further comprising the step of expanding a third portion of the compressed refrigerant flow to form a third low-temperature refrigerant flow, The method according to embodiment 16, wherein step (a) includes pre-cooling a natural gas supply flow via indirect heat exchange with first and third low-temperature refrigerant flows, and further cooling and liquefying the natural gas supply flow via indirect heat exchange with the first low-temperature refrigerant flow to form a first liquefied natural gas flow from the natural gas supply flow and heated gaseous refrigerant flows from the first and third low-temperature refrigerant flows.

[0024] Embodiment 18: The method is The method according to embodiment 17, further comprising the step of pre-cooling the first and second portions of the compressed refrigerant flow via indirect heat exchange with the first and third low-temperature refrigerant flows before the first portion of the compressed refrigerant flow is expanded in step (d) and before the second portion of the compressed refrigerant flow is further cooled in step (j).

[0025] Embodiment 19: The method according to Embodiment 17 or 18, wherein the third low-temperature refrigerant flow is a gaseous refrigerant flow.

[0026] Embodiment 20: The method according to any one of Embodiments 1 to 19, wherein the first low-temperature refrigerant flow is a gaseous refrigerant flow.

[0027] Embodiment 21: The method according to any one of Embodiments 1 to 20, wherein the natural gas supply stream is cooled in step (a) to form a first liquefied natural gas stream at a temperature of 90°C to 115°C.

[0028] Embodiment 22: A system for liquefying natural gas, One or more heat exchanger sections are arranged and configured to receive a natural gas supply flow and at least a first low-temperature refrigerant vapor, and to cool and liquefy the natural gas supply flow through indirect heat exchange with at least a first low-temperature refrigerant flow to form a first liquefied natural gas flow and a heated gaseous refrigerant flow, One or more expansion and separation devices are arranged and configured to receive, flush, and separate a first liquefied natural gas flow to form a liquefied natural gas product flow and at least a first flash gas flow. One or more conduits and refrigerant compression stages are arranged and configured to receive, combine, and compress a first flash gas flow and a heated gaseous refrigerant flow to form a compressed refrigerant flow. The system comprises an expansion device positioned and configured to receive and expand at least a first portion of a compressed refrigerant flow to form a first low-temperature refrigerant flow, A system in which the natural gas supply flow remains separated from either the first flash gas flow or the compressed refrigerant flow and is arranged and configured such that it does not combine with either the first flash gas flow or the compressed refrigerant flow. [Brief explanation of the drawing]

[0029] [Figure 1] Figure 1 is a schematic flowchart illustrating a comparative method and system for cooling and liquefying natural gas, which is not based on the present invention.

[0030] [Figure 2] Figure 2 is a schematic flowchart illustrating a method and system according to a first embodiment of the present invention for cooling and liquefying natural gas.

[0031] [Figure 2A] Figure 2A is a schematic flowchart illustrating the integrated heat exchanger and phase separator that can be used in the method and system of Figure 2. [Modes for carrying out the invention]

[0032] A method and system for liquefying natural gas is described herein, wherein a natural gas supply stream is cooled and liquefied via indirect heat exchange with one or more refrigerant streams, and the resulting LNG stream is then flushed and separated to produce LNG products.

[0033] As used herein, unless otherwise indicated, the articles “a” and “an” mean one or more features when applied to any features in embodiments of the invention described herein and in the claims. The use of “a” and “an” is not limited to a single feature unless such limitation is specifically stated. The article “the” preceding a singular or plural noun or noun phrase indicates a specific designated feature or a group of specific designated features and may have singular or plural implications depending on the context in which it is used.

[0034] Where letters are used herein to identify enumerated steps of a method (e.g., (a), (b), and (c)), these letters are used merely to aid in referring to the steps of the method and are not intended to indicate a specific order in which the claimed steps are performed, except where such order is specifically enumerated, and only insofar as such order is specifically enumerated.

[0035] Where used herein to identify enumerated features of a method or system, terms such as “first,” “second,” “third,” etc., are used solely to aid in referring to and distinguishing the features in question, and are not intended to indicate any specific order of features, except where such order is specifically enumerated, and only insofar as such order is specifically enumerated.

[0036] As used herein, the term “natural gas” also includes synthetic natural gas and / or alternative natural gases. The main component of natural gas is methane (typically present in at least 85 mol%, more often at least 90 mol%, and on average about 95 mol% of the feedstream). Other typical components of raw natural gas that may be present in smaller amounts include one or more “light components” (i.e., components with lower boiling points than methane), such as nitrogen, helium, and hydrogen, and one or more “heavy components” (i.e., components with higher boiling points than methane), such as carbon dioxide and other acidic gases, water, mercury, and heavier hydrocarbons such as ethane, propane, butane, and pentane. However, before liquefaction, the raw natural gas feedstream is treated as necessary to reduce the levels of any heavy components that may be present to levels required to avoid freezing or other operational problems in the heat exchanger section where the natural gas is cooled and liquefied.

[0037] As used herein, the term “liquefied natural gas” refers to natural gas in a liquid phase, or, in relation to natural gas at temperatures and pressures above its critical point (i.e., supercritical fluid), natural gas at a density greater than its critical point density. Similarly, references to “liquefaction” of natural gas refer to the conversion of natural gas from vapor to liquid (i.e., from a gaseous phase to a liquid phase) (typically by cooling), or, in relation to natural gas at temperatures and pressures above its critical point, the act of increasing the density of natural gas to a density greater than its critical point density (typically by cooling).

[0038] As used herein, the term “indirect heat exchange” refers to heat exchange between two fluids that remain separated from each other by some form of physical barrier.

[0039] As used herein, the term “heat exchanger section” means a unit or part of a unit in which indirect heat exchange occurs between one or more fluid flows through the cold side of the heat exchanger section and one or more fluid flows through the heated side of the heat exchanger section, the fluid flows through the cold side being heated by it, and the fluid flows through the heated side being cooled by it (the terms “heated side” and “cold side” are purely relative). Unless otherwise indicated, a heat exchanger section may be any preferred type of heat exchanger section, including, but not limited to, shell and tube heat exchanger sections, coil-wound, or plate and fin type heat exchangers.

[0040] As used herein, the terms “coil-wound heat exchanger” and “coil-wound heat exchanger unit” refer to a type of heat exchanger known in the art, comprising one or more tube bundles enclosed within a shell casing. A “coil-wound heat exchanger section” comprises one or more such tube bundles, the “tube side” of the bundles, i.e., typically representing the heated side of the section and defining one or more passages (also referred to as tube circuits) through the section, the interior of the tubes within the bundles, and the “shell side” of the bundles, i.e., typically representing the cold side of the section and defining a single passage through the section, the space between the interior of the shell casing and the exterior of the tubes, determined by the interior of the shell casing and the exterior of the tubes. The shell side is almost always used as the cold side of a section because it provides much lower flow resistance than the tube side, allows for much greater pressure drops than the tube side, and makes the expanded flow of cold refrigerant passing through the shell side much more effective and efficient, and the refrigerant provides a cooling duty to the section passing through the shell side. Coil-wound heat exchangers are compact heat exchangers known for their robustness, safety, and heat transfer efficiency, thus offering the advantage of providing a very efficient level of heat exchange for their footprint. However, because the shell side defines only a single passage through the heat exchanger section, it is not possible to use two or more refrigerant flows on the shell side of a coil-wound heat exchanger section without the flow of refrigerants being mixed on the shell side of that heat exchanger section.

[0041] As used herein, the term “flash” (also referred to in the art as “flash evaporation”) refers to the process of reducing the pressure of a liquid (or supercritical or two-phase) flow to cool the flow, evaporate a portion of the liquid, and produce a two-phase mixture of vapor and liquid at a lower temperature and lower pressure, the vapor present in this mixture also referred to as “flash gas.” As used herein, the phrase “flash and separation” refers to the process of flashing a flow and separating the flash gas from the remaining liquid.

[0042] As used herein, the terms “gaseous flow of refrigerant” and “gaseous refrigerant flow” refer to a flow of refrigerant where substantially all of the flow, more preferably all of the flow, is vapor (i.e., in the gaseous phase). Preferably, the flow is at least 80 mol% vapor (i.e., having at least 0.8 vapor fraction), and more preferably, the flow is at least 90 mol%, at least 95 mol%, or at least 99 mol% vapor.

[0043] As used herein, the term “expansion device” refers to any device or set of devices suitable for expanding and thereby reducing the pressure of a fluid. Types of expansion devices suitable for expanding a fluid include “isentropic” expansion devices, such as expanders (i.e., turbo expanders) or hydraulic turbines, in which the fluid is expanded and thereby the pressure and temperature of the fluid are reduced substantially in an isentropic manner (i.e., in a manner that produces a workpiece), and “isenthalpic” expansion devices, such as valves or other throttle devices, in which the fluid is expanded and thereby the pressure and temperature of the fluid are reduced without producing a workpiece.

[0044] As used herein, the term “separation device” refers to any device or set of devices suitable for separating a two-phase (vapor and liquid) flow or mixture into separate vapor (gas) and liquid flows. Examples of separation devices include phase separators and distillation columns. The term “distillation column” refers to a column that encompasses one or more separation stages and is comprised of devices such as packing or trays that increase contact and thus enhance mass transfer between the upward-flowing vapor and the downward-flowing liquid within the column, so that the liquid and vapor flows exiting the column are not in equilibrium (the concentration of higher volatile components increases in the upward-flowing vapor, and the concentration of lower volatile components increases in the downward-flowing liquid). The term “phase separator” refers to a drum or other form of container in which a two-phase flow can be separated into the vapor and liquid phases of its constituent components, and the liquid and vapor flows exiting the container are in equilibrium (there are no separation stages within the phase separator).

[0045] As an example, exemplary embodiments of the present invention will be described with reference to the drawings. In the drawings, if a feature is common to two or more drawings, that feature is assigned the same reference numeral. Unless a feature is specifically described as being different from other embodiments shown in the drawings, it can be assumed that the feature has the same structure and function as the corresponding feature in the embodiment in which it is described. Furthermore, in the embodiments described later, if a feature does not have a different structure or function, it may not be specifically mentioned in the specification.

[0046] Referring to Figure 1, a comparative method and system for liquefying natural gas, not relating to the present invention, is shown. The method and system shown in Figure 1 are similar to the AP-C1® method and system described in the paper "Innovative Liquefaction Technology for Floating LNG" by Roberts et al., related to the 2017 Gastech Conference & Exhibition, and in the paper "An Evolutionary Approach" by Roberts et al. (Hydrocarbon Engineering, February 2019).

[0047] The natural gas supply streams 100 and 122 are sent to a main heat exchanger comprising a pre-cooling heat exchanger section 124 and a liquefaction heat exchanger section 130. Before the natural gas supply stream 122 is introduced into the main heat exchanger, two auxiliary streams of natural gas 102 and 112 are withdrawn from the natural gas supply stream 100. The natural gas supply stream 122 is pre-cooled in the pre-cooling heat exchanger section 124, and the resulting pre-cooled natural gas supply streams 126 and 128 are then further cooled and liquefied in the liquefaction heat exchanger section 130 to form the first LNG stream 132.

[0048] A first auxiliary natural gas flow 112 is sent to a first flash gas heat exchanger 114, which includes a pre-cooling heat exchanger section and a liquefaction heat exchanger section. The first auxiliary natural gas flow 112 is pre-cooled in the pre-cooling heat exchanger section of the first flash gas heat exchanger 114 to form a pre-cooled first auxiliary natural gas flow, which is then further cooled and liquefied in the liquefaction heat exchanger section of the first flash gas heat exchanger 114 to form a second LNG flow 116.

[0049] The second auxiliary natural gas flow 102 is sent to a second flash gas heat exchanger 104, which includes a pre-cooling heat exchanger section and a liquefaction heat exchanger section. The second auxiliary natural gas flow 102 is pre-cooled in the pre-cooling heat exchanger section of the second flash gas heat exchanger 104 to form a pre-cooled second auxiliary natural gas flow, which is then further cooled and liquefied in the liquefaction heat exchanger section of the second flash gas heat exchanger 104 to form a third LNG flow 106.

[0050] The first side flow 119 of natural gas is withdrawn from the precooled first auxiliary natural gas flow before further cooling and liquefaction of the precooled first auxiliary natural gas flow in the liquefaction heat exchanger section of the first flash gas heat exchanger 114, and the second side flow 109 of natural gas is withdrawn from the precooled second auxiliary natural gas flow before further cooling and liquefaction of the precooled second auxiliary natural gas flow in the liquefaction heat exchanger section of the second flash gas heat exchanger 104. The first and second side flows 119 and 109 of natural gas are introduced into and combined with the precooled natural gas supply flow 126 before the precooled natural gas supply flow 128 is further cooled and liquefied in the liquefaction heat exchanger section 230 of the main heat exchanger.

[0051] The first LNG flow 132 is expanded in the LNG hydraulic turbine 133 before being flushed across the JT valve 135, and the second LNG flow 116 is flushed across the JT valve 117. The two flows are then combined and introduced into a high-pressure (HP) flash drum 136, where they are separated into a liquid phase and a vapor phase. The vapor phase and the liquid phase are then taken out of the HP flash drum 136 to form the first flash gas flow 137 and the fourth LNG flow 141, respectively.

[0052] The third LNG stream 106 and the fourth LNG stream 141 are flashed across JT valves 107 and 142 before being combined and introduced into the low-pressure (LP) flash drum 144, where they are separated into a liquid phase and a vapor phase. The vapor phase and the liquid phase are then taken out of the LP flash drum 144 to form the second flash gas stream 147 and the LNG product stream 145, respectively. The LNG product stream 145 is sent to the LNG storage tank 193 for storage.

[0053] The first flash gas flow 137 is sent to the low-temperature side of the first flash gas heat exchanger 114 to provide a cooling duty for pre-cooling, cooling, and liquefying the first auxiliary natural gas flow 112. The first flash gas flow 137 is heated on the low-temperature side of the first flash gas heat exchanger 114 to form a heated first flash gas flow 139.

[0054] The second flash gas flow 147 is sent to the low-temperature side of the second flash gas heat exchanger 104 to provide a cooling duty for pre-cooling, cooling, and liquefying the second auxiliary natural gas flow 102. The second flash gas flow 147 is heated on the low-temperature side of the second flash gas heat exchanger 104 to form a heated second flash gas flow 148.

[0055] Next, the heated first flash gas flow 139 and the heated second flash gas flow 148 are compressed and combined in a multi-stage flash gas compressor 149 to form a compressed flash gas flow 151, which is then cooled in an aftercooler 153. The steam accumulated in the headspace of the LNG storage tank 293 is taken out of the LNG storage tank 193 as a boil-off gas (BOG) flow 194, which is sent from the LNG storage tank 193 to a BOG compressor 195. After being compressed in the BOG compressor 195, the BOG flow is cooled in a BOG aftercooler 197. The compressed flash gas flow 155 exiting the aftercooler 153 is combined with the compressed BOG flow 199 exiting the BOG aftercooler 197 and introduced into the natural gas supply flow 100 before the natural gas supply flow 122 is introduced into the main heat exchanger.

[0056] Cooling for the main heat exchanger is provided by a methane-based refrigerant (typically consisting of natural gas with a small amount of nitrogen) circulating in a closed-loop inverse Brayton cooling cycle. Briefly, the heated gaseous refrigerant 189 exiting the cold side of the precooling section 124 of the main heat exchanger is compressed in a multistage refrigerant compressor having first 158 ​​and second 165 compression stages with an intercooler 161 and an aftercooler 168. The compressed refrigerant flow 170 exiting the aftercooler is then split into two flows 171 and 174. Flow 171 is further compressed in the compressor section of the heated compressor expander 172 to form flow 173, and flow 174 is further compressed in the compressor section of the cold compressor expander 175 to form flow 176. Flows 173 and 176 are then combined again and cooled in the aftercooler 178 to form a further compressed refrigerant flow 179.

[0057] Next, the compressed refrigerant flow 179 is further divided into two flows: a flow 181 consisting of a first portion of the compressed refrigerant, and a flow 180 consisting of a second portion of the compressed refrigerant. Flow 181 is pre-cooled in the pre-cooled heat exchanger section 124 of the main heat exchanger, and the resulting pre-cooled flow 184 is then expanded in the expander section of the cryogenic compressor expander 175 to form the first cryogenic refrigerant flow 185. Flow 180 is expanded in the expander section of the heated compressor expander 172 to form the second cryogenic refrigerant flow 187.

[0058] The first low-temperature refrigerant flow 185 is sent to the low-temperature side of the liquefaction section 130, where it is heated and provides a cooling duty for further cooling and liquefying the pre-cooled natural gas feed vapor 128. The first low-temperature refrigerant flow 186 and the second low-temperature refrigerant flow 187, exiting the liquefaction section 130, are combined and introduced to the low-temperature side of the pre-cooling section 124, where they are heated and provide a cooling duty for pre-cooling the flow 181 consisting of the natural gas feed vapor 122 and the first portion of the compressed refrigerant flow. The combined first and second refrigerant flows, exiting the pre-cooling section 124, then, as discussed above, form a heated gaseous refrigerant flow 289 that is compressed in the multi-stage refrigerant compressor 158 / 165.

[0059] Referring to Figure 2, a method and system for liquefying natural gas according to a first embodiment of the present invention are shown.

[0060] Typically, natural gas feedstreams 200, 222, at ambient temperature and high pressure, typically 20–100 bara, are fed into a main heat exchanger comprising one or more heat exchanger sections for cooling and liquefying the natural gas feedstream 222. Preferably, the natural gas feedstream 200 is at least substantially free of natural gas liquids (C2–C5+ hydrocarbons), heavy hydrocarbons (C6+ hydrocarbons), and aromatic compounds (e.g., benzene, toluene, ethylbenzene, and xylene). Typically, the natural gas feedstream 200 is already pre-treated in a pre-treatment section (not shown). Depending on the composition of the natural gas feed, the pre-treatment may involve treating the natural gas feedstream in an acidic gas removal unit for removing H2S and CO2 impurities, a dehydration unit for removing water, and / or a mercury removal unit.

[0061] Before the natural gas supply flow 222 is introduced into the main heat exchanger, two auxiliary flows of natural gas 202 and 212 are withdrawn from the natural gas supply flow 200. More specifically, the initial natural gas supply flow 200 is divided into three parts. A first part, comprising 5–40 percent, preferably 15–30 percent, of the flow of the initial natural gas supply flow 200, is withdrawn to form the first auxiliary natural gas flow 212. A second part, comprising 5–30 percent, preferably 10–20 percent, of the flow of the initial natural gas supply flow 200, is withdrawn to form the second auxiliary natural gas flow 202. Finally, a third (typically primary) part, comprising the remaining flow of the initial natural gas supply flow 200, is then sent to the main heat exchanger for cooling and liquefaction to form the natural gas flow 222 that is introduced.

[0062] In the illustrated embodiment, the main heat exchanger comprises two heat exchanger sections 224, 230, namely a pre-cooling section 224 and a liquefaction section 230. In the illustrated embodiment, both the pre-cooling section 224 and the liquefaction section 230 are coil-wound heat exchanger sections housed in separate units. However, in other embodiments, the two sections may be housed in a single unit (such as a coil-wound heat exchanger unit in which the two sections are housed in the same shell casing), and / or may be different types of heat exchanger sections, such as shell and tube or plate-fin type heat exchanger sections, although coil-wound heat exchanger sections are preferred. The main heat exchanger may also consist of only one heat exchanger section, or three or more heat exchanger sections arranged in series and / or parallel, instead of having only two heat exchanger sections. For example, in one embodiment, the pre-cooling section 224 may be divided into two or more pre-cooling sections arranged in parallel and both connected in series to the liquefaction section 230, and the flow is divided between the pre-cooling sections before being recombined.

[0063] The natural gas supply stream 222 is pre-cooled to -45°C to -25°C, more preferably -40°C to -30°C, in the pre-cooling heat exchanger section 224 via indirect heat exchange with the first and third low-temperature refrigerant streams 286 and 287, which will be described in more detail below. The resulting pre-cooled natural gas supply streams 226 and 228 are then further cooled and liquefied in the liquefaction heat exchanger section 230 via indirect heat exchange with the first low-temperature refrigerant stream 285 to form the first LNG stream 232, which is taken out of the liquefaction heat exchanger section 230 at a temperature of -115°C to -90°C, more preferably -110°C to -95°C.

[0064] The first auxiliary natural gas flow 212 is sent to the first flash gas heat exchanger 214 for cooling and liquefaction, and the second auxiliary natural gas flow 202 is sent to the second flash gas heat exchanger 204 for cooling and liquefaction.

[0065] In the illustrated embodiment, the first and second flash gas heat exchangers 214 and 204 each consist of two heat exchanger sections in the form of a pre-cooling section and a liquefaction section. In the illustrated embodiment, the pre-cooling and liquefaction sections of the first flash gas heat exchanger 214 are coil-wound heat exchanger sections housed in a single unit (i.e., within the same shell casing), and the pre-cooling and liquefaction sections of the second flash gas heat exchanger 204 are coil-wound heat exchanger sections housed in a single unit (i.e., within the same shell casing). However, in other embodiments, the two sections of each heat exchanger may be housed in separate units (e.g., separate shell casings), and / or different types of heat exchanger sections, such as shell and tube or plate-fin type heat exchanger sections, although coil-wound heat exchanger sections are preferred. Each flash gas heat exchanger may also consist of more or fewer heat exchanger sections.

[0066] The first auxiliary natural gas flow 212 is pre-cooled to -25°C to -5°C, preferably -20°C to -10°C, in the pre-cooling heat exchanger section of the first flash gas heat exchanger 214 to form a pre-cooled first auxiliary natural gas flow. The pre-cooled first auxiliary natural gas flow is then further cooled and liquefied in the liquefaction heat exchanger section of the first flash gas heat exchanger 214 to form a second LNG flow 216, which is taken out of the liquefaction heat exchanger section at a temperature of -135°C to -115°C, more preferably -130°C to -120°C. The pre-cooling, cooling, and liquefaction of the first auxiliary natural gas flow 212 in the first flash gas heat exchanger 214 are achieved via indirect heat exchange with the first flash gas flow 237, which will be described in more detail below.

[0067] The second auxiliary natural gas flow 202 is pre-cooled to -25°C to -5°C, preferably -20°C to -10°C, in the pre-cooling heat exchanger section of the second flash gas heat exchanger 204 to form a pre-cooled second auxiliary natural gas flow. The pre-cooled second auxiliary natural gas flow is then further cooled and liquefied in the liquefaction heat exchanger section of the second flash gas heat exchanger 204 to form a third LNG flow 206, which is taken out of the liquefaction heat exchanger section at a temperature of -155°C to -135°C, more preferably -150°C to -140°C. The pre-cooling, cooling, and liquefaction of the second auxiliary natural gas flow 202 in the second flash gas heat exchanger 204 are achieved via indirect heat exchange with the second flash gas flow 247, which will be described in more detail below.

[0068] In the illustrated embodiment, a first side flow 219 of natural gas is withdrawn from the precooled first auxiliary natural gas flow before further cooling and liquefaction of the precooled first auxiliary natural gas flow in the liquefaction heat exchanger section of the first flash gas heat exchanger 214, and a second side flow 209 of natural gas is withdrawn from the precooled second auxiliary natural gas flow before further cooling and liquefaction of the precooled second auxiliary natural gas flow in the liquefaction heat exchanger section of the second flash gas heat exchanger 204. The first and second side flows 219 and 209 of natural gas are introduced into and combined with the precooled natural gas supply flow 226 before the precooled natural gas supply flow 228 is further cooled and liquefied in the liquefaction heat exchanger section 230 of the main heat exchanger. This is done to better balance the cooling duty between the various heat exchanger sections. The first side flow 219 is withdrawn at a temperature of -25°C to -5°C, more preferably -20°C to -10°C, and has a flow rate of 10 to 50 percent, more preferably 20 to 40 percent, of the flow rate of the first auxiliary natural gas flow 212. The second side flow 209 is withdrawn at a temperature of -25°C to -5°C, more preferably -20°C to -10°C, and has a flow rate of 10 to 50 percent, more preferably 20 to 40 percent, of the flow rate of the second auxiliary natural gas flow 202.

[0069] The first LNG flow 232 is taken from the liquefaction section 230 of the main heat exchanger, the second LNG flow 216 is taken from the liquefaction heat exchanger section of the first flash gas heat exchanger 214, and the second low-temperature refrigerant flow 290 is taken from the liquefaction section 230 of the main heat exchanger, and the fourth LNG flow 241 and the first flash gas flow 237 are combined, flushed, and separated, as will be described in detail below.

[0070] In the illustrated embodiment, the first LNG flow 232 is expanded in the LNG hydraulic turbine 233, where the workpiece is extracted by reducing the flow pressure (therefore increasing liquefaction efficiency) before being flushed across the JT valve 235. The second cryogenic refrigerant flow 290 passes through (and expands across, if necessary) the flow control valve 291 and combines with the first LNG flow 232 upstream of the hydraulic turbine 233, or passes through and across the flow control valve 291A and combines with the first LNG flow 232 downstream of the JT valve 235, depending on whether the pressure of the second cryogenic refrigerant flow 290 is greater than or equal to the pressure of the first LNG flow 232 (in which case the steam is combined upstream of the hydraulic turbine 233) or less than the pressure of the first LNG flow 232 (in which case the steam is combined downstream of the JT valve 235). The second LNG flow 216 is flashed across the JT valve 217 and combined with the first LNG flow 232 and the second cryogenic refrigerant flow 290 (downstream of the JT valve 235). The combined flows are then introduced into a phase separator in the form of a high-pressure (HP) flash drum 236, where they are separated into a liquid phase and a vapor phase. The HP flash drum 236 operates at a pressure of 20–5 bara. The vapor phase and the liquid phase are taken out of the HP flash drum 236 to form the first flash gas flow 237 and the fourth LNG flow 241, respectively.

[0071] However, it should be noted that any suitable configuration can be used to combine, flush, and separate the first LNG flow 232, the second LNG flow 216, and the second cryogenic coolant flow 290. The LNG hydraulic turbine 233 may be omitted. The first LNG flow 232 and the second LNG flow 216 can be combined and then flushed together if they are acquired at essentially the same pressure. One or each of the first LNG flow 232, the second cryogenic refrigerant flow 290, and the second LNG flow 216 may be introduced separately into the HP flash drum 236, where the flows are then combined, or one or each of the first LNG flow 232, the second cryogenic refrigerant flow 290, and the second LNG flow 216 may be flashed and separated in their respective phase separators, the vapor phase of the phase separator being combined to form the first flash gas flow 237, and the liquid phase of the phase separator being combined to form the fourth LNG flow 241.

[0072] Next, the third LNG stream 206 and the fourth LNG stream 241, taken from the liquefaction heat exchanger section of the second flash gas heat exchanger 204, are combined, flashed, and separated to form the LNG product stream 245 and the second flash gas stream 247.

[0073] In the illustrated embodiment, the third LNG stream 206 and the fourth LNG stream 241 are combined and flashed across JT valves 207, 242 before being introduced into a phase separator in the form of a low-pressure (LP) flash drum 244, where they are separated into a liquid phase and a vapor phase. The LP flash drum 244 operates at a pressure of 10 to 1 bara. The vapor phase and the liquid phase are taken out of the LP flash drum 244 to form a second flash gas stream 247 and an LNG product stream 245, respectively.

[0074] However, it should be noted that any preferred configuration can be used to combine, flash, and separate the third LNG flow 206 and the fourth LNG flow 241. For example, the third LNG flow 206 and the fourth LNG flow 241 may be introduced separately into an LP flash drum 244, and then the two flows may be combined, or the third LNG flow 206 and the fourth LNG flow 241 may be flash-separated in their own phase separators, the vapor phases of the phase separators may be combined to form a second flash gas flow 247, and the liquid phases of the phase separators may then be combined to form an LNG product flow 245.

[0075] The first flash gas flow 237 is supplied to the low-temperature side of the first flash gas heat exchanger 214, as described above, to provide a cooling duty for pre-cooling, cooling, and liquefying the first auxiliary natural gas flow 212. The first flash gas flow 237 is heated on the low-temperature side of the first flash gas heat exchanger 214 (through indirect heat exchange with the first auxiliary natural gas flow 212 taken from the natural gas supply flow 200) to within a few degrees Celsius of the temperature of the natural gas supply flow 200, forming a heated first flash gas flow 239.

[0076] The second flash gas flow 247 is sent to the low-temperature side of the second flash gas heat exchanger 204, as described above, to provide a cooling duty for pre-cooling, cooling, and liquefying the second auxiliary natural gas flow 202. The second flash gas flow 247 is heated on the low-temperature side of the second flash gas heat exchanger 204 (through indirect heat exchange with the second auxiliary natural gas flow 202 taken from the natural gas supply flow 200) to within a few degrees Celsius of the temperature of the natural gas supply flow 200, forming a heated second flash gas flow 248.

[0077] Next, the heated first flash gas flow 239 and the heated second flash gas flow 248 are compressed and combined to form a compressed flash gas flow 255. In the illustrated embodiment, the heated first and second flash gas flows 239 and 248 may have intercooling (in the form of one or more intercoolers) to improve efficiency, for example, and are combined and compressed in a multi-stage flash gas compressor 249, where the heated second flash gas flow 248 is sent to the inlet of the flash gas compressor 249 and the heated first flash gas flow 239 is sent to an intermediate stage of the flash gas compressor 249. In this arrangement, the total head across the flash gas compressor 249 may be, for example, 25,000 to 40,000 meters. Next, the compressed flash gas flow 251 exiting the flash gas compressor 249 is cooled in the aftercooler 253 with respect to an ambient temperature fluid, such as water, to form a compressed flash gas flow 255 at ambient temperature, for example. In other embodiments, the multistage flash gas compressor 249 may be replaced by separate compressors operating, for example, in series (for example, in a similar manner to the multistage flash gas compressor) or in parallel (for example, with a heated first flash gas flow 239 and a heated second flash gas flow 248 being compressed separately and then combined).

[0078] In the illustrated embodiment, the LNG product stream 245 is sent to the LNG storage tank 293 for storage. For example, steam accumulated in the headspace of the LNG storage tank 293, consisting of tank flush, boil-off gas, and steam displacement, is taken out of the LNG storage tank 293 as a boil-off gas (BOG) stream 294. The BOG stream 294 is sent from the LNG storage tank 293 to the BOG compressor 295. After being compressed in the BOG compressor 295, the BOG stream is cooled in the BOG aftercooler 297 to an ambient temperature fluid, such as water, to form a BOG stream 299 compressed at ambient temperature, for example. Alternatively, depending on preferred operation, the LNG storage tank 293 may operate at a bubble point. In this case, the BOG flow 294, the associated BOG compressor 295, and the BOG aftercooler 297 may be omitted, or the BOG flow 294 may consist only of steam displacements for which the BOG compressor 295 and the BOG aftercooler 297 are sized accordingly.

[0079] The compressed flash gas flow 255 is combined with heated gaseous refrigerant vapor 289 exiting the low-temperature side of the pre-cooling section 224 of the main heat exchanger, and, if present, with the compressed BOG flow 299, and compressed to form the compressed refrigerant flow 270. In the illustrated embodiment, the compressed flash gas flow 255, heated gaseous refrigerant vapor 289, and compressed BOG flow 299 are combined and compressed within a multi-stage refrigerant compressor having an intercooler and an aftercooler. The compressed flash gas, heated gaseous refrigerant, and the compressed, combined flow 257 are compressed within a first compression stage 258 of the refrigerant compressor, forming the refrigerant flow 260, and then cooled within the intercooler 261 (to an ambient temperature fluid, e.g., water). Next, the refrigerant flow 263 exiting the intercooler 261 is further compressed in the second compression stage 265 of the refrigerant compressor and cooled (relative to ambient temperature fluid, such as water) in the aftercooler 268, which forms a compressed refrigerant flow 270.

[0080] The compressed refrigerant flow 270 is then divided into two flows 271 and 274 to distribute the flow between the compression stages (compressor sections) of the two compression expanders 272 and 275. Flow 271, which constitutes 40–80 percent, more preferably 50–70 percent, of the compressed refrigerant flow 270, is further compressed in the compressor section of the heated compression expander 272 to form flow 273, and flow 274 is further compressed in the compressor section of the cold compression expander 275 to form flow 276. Flows 273 and 276 are then recombined and cooled in an aftercooler 278 (relative to an ambient temperature fluid, such as water) to form a further compressed refrigerant flow 279. In an alternative configuration, flows 273 and 276 may be cooled in separate aftercoolers before being recombined.

[0081] Next, the further compressed refrigerant flow 279 is again divided into two flows, namely, a flow 281 consisting of a first and second portion of the compressed refrigerant flow (which will be further described below) and constituting 40 to 80 percent, more preferably 50 to 70 percent, of the flow rate of the compressed refrigerant flow 279, and a flow 280 consisting of a third portion of the compressed refrigerant flow 279.

[0082] Flow 281, consisting of the first and second portions of the compressed refrigerant flow, is pre-cooled to -45°C to -25°C, more preferably -40°C to -30°C, in the pre-cooling heat exchanger section 224 of the main heat exchanger via indirect heat exchange with the first and third refrigerant flows 286, 287. Flow 281 is then delivered through a separate circuit (i.e., one or more passages) on the heating side of the pre-cooling heat exchanger section 224, rather than through the circuit (i.e., one or more passages) through which the natural gas supply flow 222 passes, and is pre-cooled to a temperature similar to that of the pre-cooled natural gas supply flow 226. The resulting pre-cooled flow 282 is then further divided into flow 284, consisting of the first portion of the compressed refrigerant flow, and flow 283, consisting of the second portion of the compressed refrigerant flow.

[0083] Consisting of a second portion of the compressed refrigerant flow, flow 283, which constitutes 5–35 percent, more preferably 10–20 percent, of the flow rate of flow 282, is then further cooled (and liquefied) in the liquefied heat exchanger section 230 of the main heat exchanger via indirect heat exchange with the first refrigerant flow 285, as discussed above, and is taken out of the liquefied heat exchanger section 230 at a temperature of -115°C to -90°C, more preferably -110°C to -95°C, and then combines with the first LNG flow 232, flushes, and separates to form a second low-temperature refrigerant flow 290, through which flow 283 passes via a separate circuit on the heating side of the liquefied heat exchanger section 230, rather than through a circuit through which the pre-cooled natural gas supply flow 228 passes and is cooled to a similar temperature as the first LNG flow 232.

[0084] Flow 280, consisting of a third portion of the compressed refrigerant flow, expands in the expander section of the heated compressed expander 272 to form a third low-temperature refrigerant flow 287, which is sent to the low-temperature side of the pre-cooling section 224 of the main heat exchanger, providing a cooling duty (alongside the first low-temperature refrigerant flow) for pre-cooling the natural gas supply steam 222 and the flow 281 consisting of the first and second portions of the compressed refrigerant flow, as described above. Preferably, the third portion of the compressed refrigerant flow remains gaseous when expanded in the expander section of the heated compressed expander 272, so that the third low-temperature refrigerant flow 287 is formed as a gaseous refrigerant flow.

[0085] The flow 284, consisting of the first portion of the compressed refrigerant flow, expands in the expander section of the cryogenic compressor expander 275 to form a first cryogenic refrigerant flow 285, which is sent to the cryogenic side of the liquefaction section 230 of the main heat exchanger, providing a cooling duty for further cooling and liquefying the precooled natural gas supply steam 228 and the flow 283, consisting of the second portion of the compressed refrigerant flow, as described above. The first portion of the compressed refrigerant flow preferably remains gaseous when expanded in the expander section of the cryogenic compressor expander 275, so that the first cryogenic refrigerant flow 285 is formed as a gaseous refrigerant flow.

[0086] More specifically, the first low-temperature refrigerant flow is introduced into the low-temperature side of the liquefaction section 230 and heated, where it is heated through indirect heat exchange with the precooled natural gas supply vapor 228 and the flow 283 consisting of the second portion of the compressed refrigerant flow. The first low-temperature refrigerant flow 286, exiting the liquefaction section 230 (heated to within a few degrees Celsius of the temperature of the precooled natural gas supply flow 228 entering the liquefaction section 230), passes through the low-temperature side of the precooling section 224 alongside the third low-temperature refrigerant flow 287, where the first low-temperature refrigerant flow 286 is further heated, and the third low-temperature refrigerant flow 287 is heated through indirect heat exchange with the natural gas supply vapor 222 and the flow 281 consisting of the first and second portions of the compressed refrigerant flow. As discussed above, the combined first and third low-temperature refrigerant flows exiting the precooling section 224 (heated to within a few degrees Celsius of the temperature of the natural gas supply flow 222 entering the precooling section 224) form a heated gaseous refrigerant flow 289, which is then combined with the compressed flash gas flow 255 and, if present, the compressed BOG flow 299.

[0087] In the illustrated embodiment, the first cryogenic refrigerant flow 286 exiting the liquefaction section 230 is combined with the third cryogenic refrigerant flow 287 before the combined flow 288 is introduced to the cold side of the precooling section 224 and heated. However, in an alternative embodiment, the first cryogenic refrigerant flow 285 and the third cryogenic refrigerant flow 287 exiting the liquefaction section 230 may be introduced separately to the cold side of the precooling section 224 and combined, or (specifically, if the precooling section 224 is a plate-fin type heat exchanger section) the first cryogenic refrigerant flow 285 and the third cryogenic refrigerant flow 287 exiting the liquefaction section 230 may pass through separate passages on the cold side of the precooling section 224, be heated, and then combined after being removed from the precooling section 224.

[0088] The flash gas compressor 249, refrigerant compressors 258, 265, and (if present) the BOG compressor 295 may be powered by any preferred means. In the illustrated embodiment, a portion of the compressed flash gas flow 255 is taken out as a fuel flow 256 (before the compressed flash gas flow 255 is combined with the heated gaseous refrigerant flow 289 and, if present, the compressed BOG flow 299), and this fuel flow may be used to directly drive the compressors and / or to generate electricity used to drive the compressors. Alternatively, if power is available offsite (e.g., from the electric grid), this may be used to power the compressors, in which case there may be no need for additional fuel and the fuel flow 256 may be omitted.

[0089] As shown in Figure 2, instead of using separate flash gas heat exchangers 214 / 204 and phase separators 236 / 244, they can be replaced with an integrated heat exchanger and phase separator, as shown in Figure 2A.

[0090] In this configuration, the first flash gas heat exchanger unit 214 and the second flash gas heat exchanger unit 204 are each coil-wound heat exchanger units, each unit comprising a shell casing that encloses both the pre-cooling and liquefaction sections (in this case, both coil-wound heat exchanger sections), and a phase separator section located below the pre-cooling and liquefaction sections.

[0091] The first LNG flow 234, the second cryogenic refrigerant flow 290, and the second LNG flow 216 exiting the LNG hydraulic turbine 233 are flushed across the JT valve, combined, and introduced into the phase separator section of the first flash gas heat exchanger unit 214, where they are separated into a liquid phase and a vapor phase. The liquid phase is taken out from the bottom of the first flash gas heat exchanger unit 214 to form a fourth LNG flow 241, and the vapor phase forms a first flash gas flow that rises through the shell side of the liquefaction and precooling section of the first flash gas heat exchanger unit 214, providing a cooling duty for precooling, cooling, and liquefying the first auxiliary natural gas flow 212.

[0092] The third LNG flow 206 and the fourth LNG flow 241 flash across the JT valve, combine, and are introduced into the phase separator section of the second flash gas heat exchanger unit 204, where they are separated into a liquid phase and a vapor phase. The liquid phase is taken out from the bottom of the second flash gas heat exchanger unit 204 to form the LNG product flow 245, and the vapor phase forms a second flash gas flow that rises through the shell side of the liquefaction and precooling section of the second flash gas heat exchanger unit 204, providing a cooling duty for precooling, cooling, and liquefying the second auxiliary natural gas flow 202.

[0093] Compared to the closed-loop method and system depicted in Figure 1, and similarly the closed-loop AP-C1™ method and system described in the prior art, the “semi-open” loop method and system in Figure 2 offers improved operability and reduced equipment complexity. Specifically, in the method and system of Figure 2, the outlet of the flash gas compressor 249 is directed to the inlet of the refrigerant compressors 258 / 265 instead of connecting to the natural gas supply flow (which needs to be at a relatively high pressure for liquefaction efficiency). This shifts the cooling force from the flash gas compressor to the refrigerant compressor, allowing for a significant simplification of the flash gas compressor by enabling a reduction in the number of compression stages. For example, compared to the closed-loop AP-C1™ method and system which requires a five-stage flash gas compressor, the method in Figure 2 may require only a three-stage compressor, which would also result in one smaller compressor casing.

[0094] Compared to the open-loop method and system described and rendered in US2018 / 0180354A1, the “semi-open” loop method and system in Figure 2 can operate more efficiently, particularly during turndown. Specifically, in the method and system of US2018 / 0180354A1, a portion of the refrigerant leaving the refrigerant compressor is sent directly to the natural gas supply section, so the refrigerant compressor must be operated so that the outlet pressure of the refrigerant compressor matches the natural gas supply pressure (which, as mentioned above, needs to be a relatively high pressure for liquefaction efficiency). Conversely, in the method and system of Figure 2, the compressed refrigerant remains separated from the natural gas supply flow and is not combined with it, thereby decoupling the outlet pressure of the refrigerant compressor from the natural gas supply pressure. This makes it possible to lower the pressure of the refrigerant in the refrigerant loop, while allowing the natural gas supply flow to continue operating at a higher, more efficient liquefaction pressure, thus providing more flexibility and enabling a higher level of process optimization under different operating conditions.

[0095] Example 1 In this embodiment, Aspen Simulation Software, version 10, available from Aspen Technologies, Inc., was used to simulate a method and system for cooling and liquefying natural gas, as depicted in Figure 2.

[0096] Table 1 shows data from a simulated example. In this embodiment, the two-stage refrigerant compressors 258 / 265 have a gas horsepower of approximately 124.6 MW, the multi-stage flash gas compressor 249 (including two intercoolers in this embodiment) and the BOG compressor 295 have gas horsepowers of approximately 14.0 MW and 5.0 MW, respectively, and the second cryogenic refrigerant flow 290 is combined with the first LNG flow 232 upstream of the hydraulic turbine 233. Table 1: [Table 1A] [Table 1B] [Table 1C] [Table 1D] [Table 1E] [Table 1F] [Table 1G]

[0097] It will be understood that the present invention is not limited to the details described above with reference to preferred embodiments, and that numerous modifications and variations can be made without departing from the spirit or scope of the invention as defined in the following claims. <Note> [Form 1] A method for liquefying natural gas, (a) A step of cooling and liquefying a natural gas supply flow through indirect heat exchange with at least a first low-temperature refrigerant flow to form a first liquefied natural gas flow and a heated gaseous refrigerant flow, (b) A step of flashing and separating the first liquefied natural gas stream to form a liquefied natural gas product stream and at least a first flash gas stream, (c) A step of combining the first flash gas flow and the heated gaseous refrigerant flow and compressing them to form a compressed refrigerant flow, (d) a step of expanding at least a first portion of the compressed refrigerant flow to form the first low-temperature refrigerant flow, A method wherein the natural gas supply flow remains separated from either the first flash gas flow or the compressed refrigerant flow and is not combined with either the first flash gas flow or the compressed refrigerant flow. [Form 2] The method according to Embodiment 1, wherein step (a) is performed in one or more coil-wound heat exchanger sections of a main coil-wound heat exchanger unit or set of units. [Form 3] The method according to Embodiment 1, wherein step (c) includes compressing the first flash gas flow in one or more flash gas compression steps before combining the first flash gas flow and the heated gaseous refrigerant flow, and compressing the combined first flash gas flow and heated gaseous refrigerant flow in one or more refrigerant compression steps to form the compressed refrigerant flow. [Form 4] The method described above is (e) A step of taking a first auxiliary natural gas flow from the natural gas supply flow before the natural gas supply flow is cooled and liquefied in step (a), (f) The process further includes the step of cooling and liquefying the first auxiliary natural gas flow through indirect heat exchange with the first flash gas flow to form a second liquefied natural gas flow, The first flash gas flow is heated in step (f) before it is compressed in step (c) and combined with the heated gaseous refrigerant flow. The method according to Embodiment 1, wherein step (b) includes combining, flashing, and separating the second liquefied natural gas stream and the first liquefied natural gas stream to form the liquefied natural gas product stream and at least the first flash gas stream. [Form 5] The method according to Embodiment 4, wherein step (f) is performed in one or more coil-wound heat exchanger sections of the first flash gas heat exchanger unit or set of units. [Form 6] The method according to Embodiment 5, wherein the first flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing that encloses one or more coil-wound heat exchanger sections positioned above the phase separator section, the phase separator section being used in step (b) to separate the first flash gas flow from the first and second liquefied natural gas flows. [Form 7] Step (f) includes pre-cooling, cooling, and liquefying the first auxiliary natural gas flow via indirect heat exchange with the first flash gas flow to form a second liquefied natural gas flow, The method according to Embodiment 4, further comprising: taking a first side flow of natural gas from the first auxiliary natural gas flow after precooling the first auxiliary natural gas flow and before liquefaction; and introducing the first side flow of natural gas into the natural gas supply flow after precooling the natural gas supply flow and before liquefaction of the natural gas supply flow in step (a). [Form 8] Step (b) includes flashing and separating the first liquefied natural gas stream to form the liquefied natural gas product stream, and at least the first flash gas stream and the second flash gas stream, Step (c) includes combining and compressing the second flash gas flow, the first flash gas flow, and the heated gaseous refrigerant flow to form a compressed refrigerant flow, The aforementioned natural gas supply stream also remains separated from the second flash gas stream. The method according to Embodiment 1, which is not combined with the second flash gas flow. [Form 9] The method according to Embodiment 8, wherein step (c) comprises compressing the second flash gas flow in one or more flash gas compression steps before combining the second flash gas flow and the first flash gas flow; then compressing the combined first and second flash gas flows in one or more further flash gas compression steps before combining the combined first and second flash gas flows and the heated gaseous refrigerant flow; and compressing the combined first and second flash gas flows and the heated gaseous refrigerant flow in one or more refrigerant compression steps to form the compressed refrigerant flow. [Form 10] The method described above is (e) A step of taking a first auxiliary natural gas stream and a second auxiliary natural gas stream from the natural gas supply stream before the natural gas supply stream is cooled and liquefied in step (a), (f) A step of forming a second liquefied natural gas flow by cooling and liquefying the first auxiliary natural gas flow through indirect heat exchange with the first flash gas flow, (g) a step of cooling and liquefying the second auxiliary natural gas flow through indirect heat exchange with the second flash gas flow to form a third liquefied natural gas flow, The first flash gas flow is compressed in step (c) and heated in step (f) before being combined with the second flash gas flow and the heated gaseous refrigerant flow. The second flash gas flow is compressed in step (c) and heated in step (g) before being combined with the first flash gas flow and the heated gaseous refrigerant flow. The method according to Embodiment 8, wherein step (b) is to combine, flush, and separate the second liquefied natural gas stream and the first liquefied natural gas stream to form a fourth liquefied natural gas stream and the first flash gas stream, and then combine, flush, and separate the fourth liquefied natural gas stream and the third liquefied natural gas stream to form the liquefied natural gas product stream and at least the second flash gas stream. [Form 11] The method according to Embodiment 10, wherein step (f) is performed in one or more coil-wound heat exchanger sections of a first flash gas heat exchanger unit or set of units, and step (g) is performed in one or more coil-wound heat exchanger sections of a second flash gas heat exchanger unit or set of units. [Form 12] The first flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing that includes one or more coil-wound heat exchanger sections positioned above the phase separator section, wherein the phase separator section is used in step (b) to separate the first flash gas flow from the first and second liquefied natural gas flows. The method according to Embodiment 11, wherein the second flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing that encloses one or more coil-wound heat exchanger sections positioned above the phase separator section, the phase separator section being used in step (b) to separate the second flash gas flow from the third and fourth liquefied natural gas flows. [Form 13] Step (f) includes pre-cooling, cooling, and liquefying the first auxiliary natural gas flow via indirect heat exchange with the first flash gas flow to form a second liquefied natural gas flow, Step (g) includes pre-cooling, cooling, and liquefying the second auxiliary natural gas flow via indirect heat exchange with the second flash gas flow to form a third liquefied natural gas flow, The method according to Embodiment 10, further comprising: taking a first side flow of natural gas from the first auxiliary natural gas flow after precooling the first auxiliary natural gas flow and before liquefaction; taking a second side flow of natural gas from the second auxiliary natural gas flow after precooling the second auxiliary natural gas flow and before liquefaction; and, in step (a), introducing the first side flow of natural gas and the second side flow of natural gas into the natural gas supply flow after precooling the natural gas supply flow and before liquefaction of the natural gas supply flow. [Form 14] The method described above is (h) A step of introducing the liquefied natural gas product stream into a liquefied natural gas storage tank and storing the liquefied natural gas product in the liquefied natural gas storage tank, (i) further comprising the step of extracting a boil-off gas flow from the liquefied natural gas storage tank, Step (c) includes combining and compressing the boil-off gas flow, the first flash gas flow, and the heated gaseous refrigerant flow to form the compressed refrigerant flow, The method according to Embodiment 1, wherein the natural gas supply stream also remains separated from the boil-off gas stream and is not combined with the boil-off gas stream. [Form 15] The method according to Embodiment 14, wherein step (c) comprises compressing the boil-off gas flow in one or more boil-off gas compression steps before combining the boil-off gas flow, the first flash gas flow, and the heated gaseous refrigerant flow, and compressing the combined boil-off gas flow, the first flash gas flow, and the heated gaseous refrigerant flow in one or more refrigerant compression steps to form the compressed refrigerant flow. [Form 16] Step (d) includes expanding the first portion of the compressed refrigerant flow to form the first low-temperature refrigerant flow, Step (b) includes combining, flashing, and separating the first liquefied natural gas stream and the second low-temperature refrigerant stream to form the liquefied natural gas product stream and at least the first flash gas stream, The method described above is (j) The method according to Embodiment 1, further comprising the step of cooling a second portion of the compressed refrigerant flow through indirect heat exchange with the first low-temperature refrigerant flow to form the second cooling refrigerant flow. [Form 17] The method described above is (k) Further comprising the step of expanding the third portion of the compressed refrigerant flow to form a third low-temperature refrigerant flow, Step (a) includes pre-cooling the natural gas supply flow via indirect heat exchange with the first and third low-temperature refrigerant flows, and further cooling and liquefying the natural gas supply flow via indirect heat exchange with the first low-temperature refrigerant flow to form the first liquefied natural gas flow from the natural gas supply flow and the heated gaseous refrigerant flows from the first and third low-temperature refrigerant flows, The method described in Form 16. [Form 18] The method described above is (l) The method of Embodiment 17, further comprising the step of pre-cooling the first and second portions of the compressed refrigerant flow via indirect heat exchange with the first and third low-temperature refrigerant flows before the first portion of the compressed refrigerant flow is expanded in step (d) and before the second portion of the compressed refrigerant flow is further cooled in step (j). [Form 19] The method according to Embodiment 1, wherein the first low-temperature refrigerant flow is a gaseous refrigerant flow. [Form 20] A system for liquefying natural gas, One or more heat exchanger sections are arranged and configured to receive a natural gas supply flow and at least a first low-temperature refrigerant vapor, and to cool and liquefy the natural gas supply flow through indirect heat exchange with at least the first low-temperature refrigerant flow to form a first liquefied natural gas flow and a heated gaseous refrigerant flow, One or more expansion and separation devices are arranged and configured to receive, flush, and separate the first liquefied natural gas flow to form a liquefied natural gas product flow and at least a first flash gas flow, One or more conduits and refrigerant compression stages are arranged and configured to receive, combine, and compress the first flash gas flow and the heated gaseous refrigerant flow to form a compressed refrigerant flow, The system comprises an expansion device that receives and expands at least a first portion of the compressed refrigerant flow to form the first low-temperature refrigerant flow, A system in which the natural gas supply flow remains separated from either the first flash gas flow or the compressed refrigerant flow and is arranged and configured such that it is not combined with either the first flash gas flow or the compressed refrigerant flow.

Claims

1. A method for liquefying natural gas, (a) A step of cooling and liquefying a natural gas supply flow through indirect heat exchange with at least a first low-temperature refrigerant flow to form a first liquefied natural gas flow and a heated gaseous refrigerant flow, (b) A step of flashing and separating the first liquefied natural gas stream to form a liquefied natural gas product stream and at least a first flash gas stream and a second flash gas stream, (c) A step of combining the first and second flash gas flows within the flash gas compressor, compressing the combined first and second flash gas flows, and combining and compressing the heated gaseous refrigerant flow with the combined first and second flash gas flows to form a compressed refrigerant flow, in which case the second flash gas flow is sent to the inlet of the flash gas compressor and the first flash gas flow is sent to an intermediate stage of the flash gas compressor, (d) a step of expanding at least a first portion of the compressed refrigerant flow to form the first low-temperature refrigerant flow, A method wherein the natural gas supply flow remains separated from either the first flash gas flow or the compressed refrigerant flow and is not combined with either the first flash gas flow or the compressed refrigerant flow.

2. The method according to claim 1, wherein step (a) is performed in one or more coil-wound heat exchanger sections of a main coil-wound heat exchanger unit or set of units.

3. The method described above is (e) A step of taking a first auxiliary natural gas flow from the natural gas supply flow before the natural gas supply flow is cooled and liquefied in step (a), (f) a step of cooling and liquefying the first auxiliary natural gas flow through indirect heat exchange with the first flash gas flow to form a second liquefied natural gas flow, The first flash gas flow is compressed in step (c) and heated in step (f) before being combined with the second flash gas flow and the heated gaseous refrigerant flow. The method according to claim 1, wherein step (b) includes combining, flashing, and separating the second liquefied natural gas stream and the first liquefied natural gas stream to form the liquefied natural gas product stream and at least the first flash gas stream and the second flash gas stream.

4. The method according to claim 3, wherein step (f) is performed in one or more coil-wound heat exchanger sections of a first flash gas heat exchanger unit or set of units.

5. The method according to claim 4, wherein the first flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing that encloses one or more coil-wound heat exchanger sections positioned above the phase separator section, the phase separator section being used in step (b) to separate the first flash gas flow from the first and second liquefied natural gas flows.

6. Step (f) includes pre-cooling, cooling, and liquefying the first auxiliary natural gas flow via indirect heat exchange with the first flash gas flow to form a second liquefied natural gas flow, The method according to claim 3, further comprising: taking a first side flow of natural gas from the first auxiliary natural gas flow after precooling the first auxiliary natural gas flow and before liquefaction; and introducing the first side flow of natural gas into the natural gas supply flow after precooling the natural gas supply flow and before liquefaction of the natural gas supply flow in step (a).

7. The method according to claim 1, wherein step (c) comprises compressing the second flash gas flow in one or more flash gas compression stages of the flash gas compressor before combining the second flash gas flow and the first flash gas flow; then compressing the combined first and second flash gas flows in one or more further flash gas compression stages of the flash gas compressor before combining the combined first and second flash gas flows and the heated gaseous refrigerant flow; and compressing the combined first and second flash gas flows and the heated gaseous refrigerant flow in one or more refrigerant compression stages to form the compressed refrigerant flow.

8. The method described above is (e) A step of taking out a first auxiliary natural gas stream and a second auxiliary natural gas stream from the natural gas supply stream before the natural gas supply stream is cooled and liquefied in step (a), (f) A step of forming a second liquefied natural gas flow by cooling and liquefying the first auxiliary natural gas flow through indirect heat exchange with the first flash gas flow, (g) a step of cooling and liquefying the second auxiliary natural gas flow through indirect heat exchange with the second flash gas flow to form a third liquefied natural gas flow, The first flash gas flow is compressed in step (c) and heated in step (f) before being combined with the second flash gas flow and the heated gaseous refrigerant flow. The second flash gas flow is compressed in step (c) and heated in step (g) before being combined with the first flash gas flow and the heated gaseous refrigerant flow. The method according to claim 1, wherein step (b) is to combine, flush, and separate the second liquefied natural gas stream and the first liquefied natural gas stream to form a fourth liquefied natural gas stream and the first flash gas stream, and then combine, flush, and separate the fourth liquefied natural gas stream and the third liquefied natural gas stream to form the liquefied natural gas product stream and at least the second flash gas stream.

9. The method according to claim 8, wherein step (f) is performed in one or more coil-wound heat exchanger sections of a first flash gas heat exchanger unit or set of units, and step (g) is performed in one or more coil-wound heat exchanger sections of a second flash gas heat exchanger unit or set of units.

10. The first flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing that includes one or more coil-wound heat exchanger sections positioned above the phase separator section, wherein the phase separator section is used in step (b) to separate the first flash gas flow from the first and second liquefied natural gas flows. The method according to claim 9, wherein the second flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing that encloses one or more coil-wound heat exchanger sections positioned above the phase separator section, the phase separator section being used in step (b) to separate the second flash gas flow from the third and fourth liquefied natural gas flows.

11. Step (f) includes pre-cooling, cooling, and liquefying the first auxiliary natural gas flow via indirect heat exchange with the first flash gas flow to form a second liquefied natural gas flow, Step (g) includes pre-cooling, cooling, and liquefying the second auxiliary natural gas flow via indirect heat exchange with the second flash gas flow to form a third liquefied natural gas flow, The method according to claim 8, further comprising: taking a first side flow of natural gas from the first auxiliary natural gas flow after precooling the first auxiliary natural gas flow and before liquefaction; taking a second side flow of natural gas from the second auxiliary natural gas flow after precooling the second auxiliary natural gas flow and before liquefaction; and, in step (a), introducing the first side flow of natural gas and the second side flow of natural gas into the natural gas supply flow after precooling the natural gas supply flow and before liquefaction of the natural gas supply flow.

12. The method described above is (h) A step of introducing the liquefied natural gas product stream into a liquefied natural gas storage tank and storing the liquefied natural gas product in the liquefied natural gas storage tank, (i) further comprising the step of extracting a boil-off gas flow from the liquefied natural gas storage tank, Step (c) includes, within the flash gas compressor, where the second flash gas flow is sent to the inlet of the flash gas compressor and the first flash gas flow is sent to an intermediate stage of the flash gas compressor, the first and second flash gas flows are combined, the combined first and second flash gas flows are compressed, and the boil-off gas flow, the heated gaseous refrigerant flow, and the combined first and second flash gas flows are combined and compressed to form the compressed refrigerant flow, The method according to claim 1, wherein the natural gas supply stream also remains separated from the boil-off gas stream and is not combined with the boil-off gas stream.

13. The method according to claim 12, wherein step (c) comprises compressing the boil-off gas flow in one or more boil-off gas compression steps before combining the boil-off gas flow, the heated gaseous refrigerant flow, and the combined first and second flash gas flows, and compressing the combined boil-off gas flow, the heated gaseous refrigerant flow, and the combined first and second flash gas flows in one or more refrigerant compression steps to form the compressed refrigerant flow.

14. Step (d) includes expanding the first portion of the compressed refrigerant flow to form the first low-temperature refrigerant flow, Step (b) includes combining, flashing, and separating the first liquefied natural gas stream and the second low-temperature refrigerant stream to form the liquefied natural gas product stream and at least the first flash gas stream and the second flash gas stream, The method described above is (j) The method according to claim 1, further comprising the step of cooling a second portion of the compressed refrigerant flow through indirect heat exchange with the first low-temperature refrigerant flow to form the second low-temperature refrigerant flow.

15. The method described above is (k) further comprising the step of expanding a third portion of the compressed refrigerant flow to form a third low-temperature refrigerant flow, Step (a) includes pre-cooling the natural gas supply flow via indirect heat exchange with the first and third low-temperature refrigerant flows, and further cooling and liquefying the natural gas supply flow via indirect heat exchange with the first low-temperature refrigerant flow to form the first liquefied natural gas flow from the natural gas supply flow and the heated gaseous refrigerant flows from the first and third low-temperature refrigerant flows, The method according to claim 14.

16. The method described above is The method according to claim 15, further comprising the step of pre-cooling the first and second portions of the compressed refrigerant flow via indirect heat exchange with the first and third low-temperature refrigerant flows before the first portion of the compressed refrigerant flow is expanded in step (d) and before the second portion of the compressed refrigerant flow is further cooled in step (j).

17. The method according to claim 1, wherein the first low-temperature refrigerant flow is a gaseous refrigerant flow.

18. A system for liquefying natural gas, One or more heat exchanger sections are arranged and configured to receive a natural gas supply flow and at least a first low-temperature refrigerant vapor, and to cool and liquefy the natural gas supply flow through indirect heat exchange with at least a first low-temperature refrigerant flow to form a first liquefied natural gas flow and a heated gaseous refrigerant flow, One or more expansion and separation devices are arranged and configured to receive, flash, and separate the first liquefied natural gas flow to form a liquefied natural gas product flow and at least a first flash gas flow and a second flash gas flow, A flash gas compressor configured to receive and combine the first and second flash gas flows and to compress the combined first and second flash gas flows, wherein the second flash gas flow is sent to the inlet of the flash gas compressor and the first flash gas flow is sent to an intermediate stage of the flash gas compressor, One or more conduits and refrigerant compression stages are arranged and configured to receive, combine, and compress the heated gaseous refrigerant flow and the combined first and second flash gas flows to form a compressed refrigerant flow. The system includes an expansion device that receives and expands at least a first portion of the compressed refrigerant flow to form the first low-temperature refrigerant flow, A system in which the natural gas supply flow remains separated from either the first flash gas flow or the compressed refrigerant flow and is arranged and configured such that it is not combined with either the first flash gas flow or the compressed refrigerant flow.

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