Boil-off gas reliquefaction system with dual cooling sources

Abstract

A reliquefaction system including a compressor configured to extract Boil-Off Gas (BOG) from a LNG tank and provide at least a portion of the BOG to a heat exchanger to be used as a first cooling source, compress the BOG exiting the heat exchanger, and deliver at least a portion of the compressed BOG to be reliquefied to the heat exchanger. A refrigeration loop is configured to provide a second cooling source to the heat exchanger, wherein the first cooling source and the second cooling source drive a cooling process in the heat exchanger to reliquefy the BOG. The system may include valves separating the BOG into multiple streams based on gas consumer load requirements, with one stream serving as the first cooling source and another stream being bypassed, mixed with BOG stream downstream of heat exchanger, compressed and subsequently separated into streams for gas consumers and reliquefaction.

Description

00243.0032US02BOIL-OFF GAS RELIQUEFACTION SYSTEM WITH DUAL COOLING SOURCES CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Application No.63 / 755,685, filed February 7, 2025, and entitled "Gas Liquefaction for LNGBoil Off Gas”, which is hereby incorporated by reference in its entirety.FIELD OF TECHNOLOGY

[0002] The present disclosure relates to liquefied natural gas processing systems, and more particularly to a boil-off gas reliquefaction system that utilizes dual cooling sources comprising both boil-off gas and a refrigeration loop to reliquefy boil-off gas from LNG tanks.BACKGROUND

[0003] Liquefied Natural Gas (LNG) has emerged as a viable energy source in the transition from fossil fuels to renewables, with LNG carriers and LNG-fueled vessels playing major roles in the evolving global energy mix. During storage and transport of LNG in cryogenic tanks aboard ships, natural heat ingress into the tanks causes tank pressure to rise due to boil-off gas generation.SUMMARY

[0004] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0005] An aspect relates to a reliquefaction system comprising a compressor configured to extract a Boil-Off Gas (BOG) from a Liquefied Natural Gas (LNG) tank and provide at least a portion of the BOG to a heat exchanger to be used as a first cooling source, compress the BOG exiting the heat exchanger, and deliver at least portion of the compressed BOG to be reliquefied to the heat exchanger. The system includes a refrigeration loop configured to provide a second cooling source to the heat exchanger, wherein the first cooling source and the second cooling source drive a cooling process in the heat exchanger to reliquefy the BOG.

[0006] In an exemplary embodiment, the reliquefaction system further comprises a first valve upstream of the compressor and the heat exchanger, which separates the BOG into a first BOG stream and a second BOG stream based on a gas consumer load requirement, wherein the first BOG stream is provided to the heat exchanger as the first cooling source and the second BOG stream is provided to the compressor to form a mixture with the BOG exiting the heat exchanger.00243.0032US02The reliquefaction system further comprises a second valve downstream of the compressor and upstream of the heat exchanger, which separates the mixture into a third BOG stream fed to a gas consumer according to the gas consumer load requirement, and a fourth BOG stream to be delivered to the heat exchanger for reliquefaction.

[0007] In an exemplary embodiment, the first BOG stream is heated by the heat exchanger and then mixed with the second BOG stream to form the mixture, which has a temperature lower than the first BOG stream downstream of the heat exchanger. A flowrate of the second BOG stream may be matched with a flowrate of the third BOG stream to optimize a pressure drop in the heat exchanger, or a flowrate of the second BOG stream can be a function of the flowrate of the third BOG stream to optimize a pressure drop in the heat exchanger.

[0008] In an exemplary embodiment, the first BOG stream is heated by the heat exchanger and then mixed with the second BOG stream to form the mixture, which has a temperature lower than the first BOG stream downstream of the heat exchanger. A flowrate of the second BOG stream is a function of load required by the gas consumer (e.g. engine) to optimize a pressure drop in the heat exchanger.

[0009] In an exemplary embodiment, the refrigeration loop comprises at least one refrigerant compressor and at least one Expander-Compressor. The refrigeration loop may also comprise a plurality of refrigerant compressors in parallel. The refrigeration loop may also comprise a plurality of Expander-Compressors and at least one refrigerant compressor. The at least one refrigerant compressor delivers a pressurized refrigerant stream to the heat exchanger, the pressurized refrigerant stream being cooled by the heat exchanger and then expanded by the at least one Expander-Compressor to provide the second cooling source to the heat exchanger. The refrigerant comprises methane gas, or a combination of methane gas and BOG, or a mixture of methane and nitrogen gas.

[0010] In an exemplary embodiment, a compressor core of the compressor is a same type of compressor core as a refrigerant compressor used in the refrigeration loop.

[0011] In an exemplary embodiment, the reliquefaction system further comprises a Joules Thompson (JT) valve configured to expand a liquefied gas exiting the heat exchanger, and a flash drum for receiving the liquefied gas, wherein the liquefied gas is returned to the LNG tank from the flash drum. A mass flow rate measurement device is in fluid communication with the flash drum to monitor a mass flow rate of a vent gas exiting the flash drum, wherein, in response to the mass flow rate of the vent gas exceeding a predetermined threshold, an operation of the compressor is changed.00243.0032US02

[0012] In an exemplary embodiment, the heat exchanger is a Brazed Aluminum Heat Exchanger (BAHX) or a Stainless Steel Welded heat exchanger. The heat exchanger is a multistream heat exchanger, and is configured to receive the first BOG stream and increase a temperature of the first BOG stream, receive the fourth BOG stream and decrease a temperature of the fourth BOG stream, receive a pressurized refrigerant stream from at least one refrigerant compressor of the refrigeration loop and decrease a temperature of the pressurized refrigerant stream, and receive an expanded refrigerant stream from an at least one Expander-Compressor of the refrigeration loop and increase a temperature of the expanded refrigerant stream.

[0013] Another aspect relates to a reliquefaction method comprising extracting BOG from a LNG tank, determining a gas consumer load requirement, in response to the gas consumer load requirement exceeding a minimum threshold, separating the BOG according to the gas consumer load requirement into a first BOG stream that flows to a heat exchanger to be used as a first cooling source and a second BOG stream that satisfies the gas consumer load requirement, which flows directly to a compressor, mixing the first BOG stream and the second BOG stream to form a mixture of the first BOG stream and the second BOG stream that is compressed by the compressor, separating the compressed mixture into a third BOG stream to be delivered to a gas consumer and a fourth BOG stream to be delivered to the heat exchanger for reliquefaction, and reliquefying the fourth BOG stream using the first cooling source and a second cooling source provided by a refrigeration loop.

[0014] In an exemplary embodiment, the refrigeration loop comprises at least one refrigerant compressor and at least one Expander-Compressor. The method further comprises expanding, by the expander compressor, a pressurized refrigerant stream pressurized by the at least one refrigerant compressor and cooled by the heat exchanger to deliver the second cooling source to the heat exchanger. The liquefied gas exiting the heat exchanger is expanded using a Joules Thompson (JT) valve and the liquefied gas is collected in a flash drum, and the liquefied gas is returned to the LNG tank from the flash drum.

[0015] Another aspect relates to a reliquefaction method comprising extracting BOG from a LNG tank, determining that a gas consumer load requirement is at or below a minimum threshold, sending the BOG to a heat exchanger to be used as a first cooling source, which flows to a compressor downstream of the heat exchanger that compresses the gas exiting the heat exchanger, separating the compressed gas into a consumer stream to be delivered to a gas consumer and a reliquefaction stream to be delivered to the heat exchanger for reliquefaction, and reliquefying the reliquefaction stream using the first cooling source and a second cooling source provided by a refrigeration loop.00243.0032US02

[0016] Another aspect relates to a computer-implemented reliquefaction method comprising extracting a Boil-Off Gas (BOG) from a Liquefied Natural Gas (LNG) tank, determining a gas consumer load requirement, in response to the gas consumer load requirement exceeding a minimum threshold, separating the BOG according to the gas consumer load requirement into a first BOG stream that flows to a heat exchanger to be used as a first cooling source and a second BOG stream, which flows directly to a compressor, mixing the first BOG stream and the second BOG stream to form a mixture of the first BOG stream and the second BOG stream that is compressed by the compressor, separating the compressed mixture into a third BOG stream to be delivered to a gas consumer and a fourth BOG stream to be delivered to the heat exchanger for reliquefaction, and reliquefying the fourth BOG stream using the first cooling source and a second cooling source provided by a refrigeration loop.

[0017] Another aspect relates to a computer-implemented reliquefaction method comprising extracting a Boil-Off Gas (BOG) from a Liquefied Natural Gas (LNG) tank, determining that a gas consumer load requirement is at or below a minimum requirement, sending the BOG to a heat exchanger to be used as a first cooling source, which flows to a compressor downstream of the heat exchanger that compresses the gas exiting the heat exchanger, separating the compressed gas into a consumer stream to be delivered to a gas consumer and a reliquefaction stream to be delivered to the heat exchanger for reliquefaction, and reliquefying the reliquefaction stream using the first cooling source and a second cooling source provided by a refrigeration loop.

[0018] The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.BRIEF DESCRIPTION OF FIGURES

[0019] Non-limiting and non-exhaustive examples are described with reference to the following figures.

[0020] FIG. 1 illustrates a hierarchical diagram of existing reliquefaction systems for processing boil-off gas, showing the classification of LNGtank pressure management technologies into BOG reliquefaction and subcooling approaches.

[0021] FIG. 2 illustrates a system diagram of a reliquefaction system with component connections, according to aspects of the present disclosure.

[0022] FIG. 3 illustrates a flowchart for a reliquefaction method that processes boil-off gas through a dual cooling source system, according to a first scenario.00243.0032US02

[0023] FIG. 4 illustrates a flowchart for a reliquefaction method that processes boil-off gas through a dual cooling source system, according to a second scenario.

[0024] FIG. 5 illustrates a flowchart for a refrigeration loop operation method, according to aspects of the present disclosure.

[0025] FIG. 6 illustrates a composite temperature profile graph that demonstrates thermal performance characteristics of a first type of heat exchanger in a reliquefaction system, according to aspects of the present disclosure.

[0026] FIG. 7 illustrates a composite temperature profile graph that demonstrates thermal performance characteristics of a second type of heat exchanger in a reliquefaction system, according to aspects of the present disclosure.DETAILED DESCRIPTION

[0027] The following description sets forth exemplary aspects of the present disclosure. It should be recognized, however, that such description is not intended as a limitation on the scope of the present disclosure. Rather, the description also encompasses combinations and modifications to those exemplary aspects described herein.

[0028] Liquefied Natural Gas (LNG) systems may generate boil-off gas (BOG) due to natural heat ingress into LNG storage tanks. The BOG generation may cause tank pressure to rise, creating operational challenges for LNG carrier vessels and LNG-fueled ships. Managing BOG may involve reliquefaction processes that convert the gaseous phase back to liquid form for return to the storage tank.

[0029] The treatment of boil-off gas presents challenges for reasons of safety, environmental compliance, and economic considerations. Tank pressure control, emission control, and the value of LNG make boil-off gas reliquefaction one of the key aspects of fuel gas systems on these vessels. LNG containment technology continues to evolve, with tank insulation improving and natural boil-off rates reducing. Prime mover technology has also seen changes, with various engine types affecting the fuel gas systems and reliquefaction systems adopted by ships using LNG as fuel or cargo.

[0030] Tightening International Maritime Organization regulations also influence system design. As a result, reliquefaction systems must accommodate various operational scenarios and trade-offs. Different types of reliquefaction systems exist, including partial reliquefaction systems used in parallel with boil-off gas consumers, and full reliquefaction systems for situations when boil-off gas demand is low.00243.0032US02

[0031] Reliquefaction systems can be classified based on whether they use refrigerant loops or operate as flow-through systems. Systems using refrigerants may employ nitrogen, heliumnitrogen mixtures, mixed refrigerants, or the boil-off gas itself as refrigerant. Flow-through systems liquefy boil-off gas through compression-expansion processes using turbines and expansion valves.

[0032] System evaluation typically considers capital expenditure, efficiency measured as energy consumption per unit of liquid formed, and operational simplicity and flexibility. High operating pressures can increase costs, while maintaining additional refrigerant inventory adds complexity. Heat exchanger selection affects startup procedures and operational constraints, with different heat exchanger types offering various trade-offs between performance, cost, and operational flexibility.

[0033] FIG. 1 illustrates a hierarchical classification system for LNG tank pressure management technologies. The diagram presents a systematic categorization that divides pressure management approaches into two primary branches: BOG Reliquefaction and Subcooling methods.

[0034] The BOG Reliquefaction branch may be further subdivided into two distinct technological approaches. The first approach utilizes Refrigeration-based systems, which may employ various refrigerant cycles and cooling mechanisms to achieve boil-off gas reliquefaction. The second approach encompasses Flow- Through systems, which may process boil-off gas through direct compression and expansion cycles without requiring separate refrigerant loops.

[0035] The Subcooling branch may represent an alternative pressure management strategy that operates through indirect cooling methods. This approach may utilize Refrigeration technology to subcool the LNG thereby reducing the rate of boil-off gas generation within the storage tank.

[0036] The hierarchical structure in FIG. 1 may demonstrate the relationship between different technical methodologies and their implementation approaches. The classification may assist in understanding the various options available for LNG tank pressure control, with each branch representing different operational principles and system configurations. The diagram may provide a framework for evaluating different reliquefaction technologies based on their fundamental operating mechanisms and design philosophies.

[0037] The categorization shown in FIG. 1 may reflect the evolution of LNG tank pressure management systems, where different approaches have been developed to address varying operational requirements, efficiency targets, and system complexity considerations. The distinction between direct BOG processing methods and indirect subcooling methods may00243.0032US02highlight the different strategies available for maintaining optimal tank pressure conditions in LNG storage and transport applications.

[0038] BOG reliquefaction and LNG subcooling systems may address safety considerations related to tank pressure control, environmental concerns regarding emission management, and economic factors associated with LNG value preservation. The systems may extract BOG from LNG tanks and process the extracted gas through various thermal and mechanical operations to achieve BOG reliquefaction or extract LNG from tank, subcool the extracted LNG and reinject the subcooled LNG to the tank.

[0039] Modern LNG containment technologies may feature improved tank insulation designs that reduce natural boil-off rates. Prime mover technologies may include various engine types that consume BOG as fuel, creating different operational scenarios for reliquefaction system design. The systems may operate in partial reliquefaction modes when gas consumers are present or in full reliquefaction modes when gas consumption demand is low.

[0040] Reliquefaction systems may incorporate heat exchange processes, compression and expansion operations, and refrigeration cycles to achieve the thermal conditions for gas-to-liquid phase conversion. The systems may utilize multiple cooling sources and process streams to optimize the reliquefaction efficiency while managing system complexity and operational flexibility.

[0041] In brief overview, embodiments of the present invention provide a reliquefaction system that utilizes dual cooling sources to efficiently convert boil-off gas back to liquid form for return to LNG storage tanks. The system may comprise a compressor configured to extract boil-off gas from an LNG tank and manage the flow of gas through the system, providing at least a portion of the extracted gas to a heat exchanger where it serves as a first cooling source. The compressor compresses the boil-off gas after it exits the heat exchanger and delivers the compressed gas back to the heat exchanger for reliquefaction processing.

[0042] A refrigeration loop provides a second cooling source to the heat exchanger, creating a dual cooling mechanism. The combination of the first cooling source from the boil-off gas itself and the second cooling source from the refrigeration loop may drive the cooling process within the heat exchanger to achieve effective reliquefaction of the boil-off gas.

[0043] The reliquefaction system may incorporate flow control mechanisms that separate the extracted boil-off gas into multiple streams based on operational requirements such as gas consumer load demands. These flow control features may enable the system to balance between supplying gas to consumers and processing gas for reliquefaction, providing operational flexibility for different scenarios encountered in LNG storage and transport applications.00243.0032US02

[0044] The heat exchanger may be configured as a multi-stream unit that simultaneously processes various gas streams at different temperature and pressure conditions, facilitating thermal exchange operations between the boil-off gas streams and refrigerant streams. The system may also include downstream processing components such as expansion valves and collection vessels that complete the reliquefaction cycle by returning the liquefied gas to the LNG tank while managing any residual vapor components.

[0045] FIG. 2 illustrates a reliquefaction system 200 according to embodiments of the present invention. The reliquefaction system 200 is configured to operate in at least two different scenarios. In a first scenario, a gas consumer load requirement exceeds a minimum threshold, and the BOG exiting the LNG tank 208 is separated into a first BOG stream 1 and a second BOG stream 2. In a second scenario, a gas consumer load requirement is at or below a minimum threshold, and the BOG exiting the LNG tank 208 is sent only to the first BOG stream 1 to maximize the first cooling source for the heat exchanger 202.

[0046] In the first scenario, the reliquefaction system 200 processes boil-off gas from an LNG tank 208. The LNG tank 208 may serve as a storage vessel for liquefied natural gas and may generate boil-off gas due to heat ingress. The LNG tank 208 may maintain the liquefied natural gas at cryogenic temperatures, and thermal energy transfer from the surrounding environment may cause a portion of the liquid to vaporize, creating the boil-off gas.

[0047] A BOG stream 218 may be extracted from the LNG tank 208 through extraction processes that remove the gaseous phase from the tank's vapor space. For instance, operation of the compressor 204 draws the BOG stream 218 through the system 200. The extraction may maintain tank pressure within operational limits while providing at least a part of boil-off gas for processing through the reliquefaction system. The BOG stream 218 may contain methane as a primary component along with other hydrocarbon gases and trace amounts of non-condensable components such as nitrogen.

[0048] A first valve 222 may be positioned upstream of other system components to control the initial separation of the extracted boil-off gas. The first valve 222 may separate the BOG stream 218 into one or more (e.g. two) flow paths based on operational parameters. In some cases, the first valve 222 may divide the boil-off gas flow according to gas consumer load requirements, directing portions of the gas to different processing pathways within the reliquefaction system.

[0049] The BOG drawn though the first valve 222 results in a first BOG stream 1 and a second BOG stream 2 from the extracted boil-off gas 218. The separation may be controlled to optimize system performance by allocating appropriate gas quantities to different system functions. The first BOG stream 1 may be directed toward heat exchange operations where the gas may serve as00243.0032US02a first cooling source, while the second BOG stream 2 may be routed to compression processes (i.e. to compressor 204) for further processing. In an exemplary embodiment, the BOG stream 218 is separated into the first BOG stream 1 and the second BOG stream 2 when it is determined that the gas consumer load exceeds a minimum threshold. As an example, if the total flowrate of the BOG stream 218 is 3200 kg / hr, the minimum required gas consumer load is 700 kg / hr, and the actual gas consumer load requirement is 1000 kg / hr, then the amount to be reliquefied is 2200 kg / hr. In the first scenario under these conditions, 2900 kg / hr of the 3200 kg / hr of the BOG stream 218 is sent to BOG stream 1 and 300 kg / hr is sent to the second BOG stream 2 because the reliquefaction load is less than an operational capacity of the heat exchanger 202.

[0050] The first BOG stream 1 is provided to a heat exchanger 202 where the first BOG stream 1 serves as a first cooling source for the reliquefaction process. The first BOG stream 1 flows through the heat exchanger 202 and undergoes thermal processing that increases a temperature of the first BOG stream 1. The first BOG stream 1 may be heated by the heat exchanger 202 as the first BOG stream 1 provides cooling capacity to other process streams within the heat exchanger 202.

[0051] A mixing valve 226 is positioned upstream of the compressor 204 and downstream of the heat exchanger 202 to combine the first BOG stream 1 and the second BOG stream 2 after the first BOG stream 1 exits the heat exchanger 202. The mixing valve 226 creates a mixture of the first BOG stream 1 and the second BOG stream 2 that may be delivered to the compressor 204 for compression processing. The mixture may have a temperature that is lower than the temperature of the first BOG stream 1 downstream of the heat exchanger 202, which provides operational advantages for compression efficiency.

[0052] The second BOG stream 2 is provided to the mixing valve 226 to combine with BOG stream 1. The mixture of BOG stream 1 and BOG stream 2 is provided to the compressor 204 for compression. The second BOG stream 2 flows to the compressor 204 without passing through the heat exchanger 202. Flow sent to the second BOG stream 2 is determined based on a gas consumer load.

[0053] The separation of the BOG stream 218 by the first valve 222 may be controlled according to gas consumer load requirements. In some cases, when the gas consumer load requirement exceeds a minimum threshold, the first valve 222 may separate the BOG stream 218 into the first BOG stream 1 and the second BOG stream 2 based on the operational demands of the system. The first BOG stream 1 may flow to the heat exchanger 202 to be used as the first cooling source, while the second BOG stream 2 may flow to the second valve 226 to be mixed with the first BOG stream 1 and compressed by the compressor 204.00243.0032US02

[0054] The flowrate of the second BOG stream 2 may be matched with a flowrate of a third BOG stream 3 to optimize pressure drop characteristics within the heat exchanger 202. In some cases, the flowrate matching may reduce pressure losses and improve system efficiency by balancing the gas flow distribution throughout the reliquefaction system. The flowrate control may also be adjusted based on operational parameters to maintain optimal performance conditions. Alternatively, the flowrate of the second BOG stream 2 may be determined as a function of a gas consumer load, such as an engine load and / or auxiliary load.

[0055] In the second scenario, in which all of the BOG is provided to BOG stream 1 to maximize the first cooling source, the gas consumer load is satisfied by a portion of compressed gas downstream of the heat exchanger 202. The reliquefaction system 200 processes boil-off gas from the LNG tank 208 as described above. The BOG stream 218 flows to the first valve 222 which directs the BOG stream 218 only to BOG stream 1. In an exemplary embodiment, the entire BOG stream 218 is provided to the BOG stream 1 for maximining cooling capacity of the first cooling source when it is determined that the gas consumer load is at or below a minimum threshold. As an example, if the total flowrate of the BOG stream 218 is 3200 kg / hr, the gas consumer load minimum threshold is 700 kg / hr, and the actual gas consumer load requirement is 700 kg / hr, then the amount to be reliquefied is 2500 kg / hr. In the second scenario, the full 3200 kg / hr can be sent to BOG stream 1 because the required gas reliquefaction will be satisfied while maximizing the cooling capacity of the first cooling source.

[0056] The first BOG stream 1 is directed toward heat exchange operations where the gas may serve as the first cooling source, which, as a result of the entire BOG stream 218 being directed to the heat exchanger 202 via BOG stream 1, has a greater cooling capacity than the first scenario. The first BOG stream 1 flows through the heat exchanger 202 and undergoes thermal processing that increases a temperature of the first BOG stream 1. In some cases, the first BOG stream 1 may be heated by the heat exchanger 202 as the first BOG stream 1 provides cooling capacity to other process streams within the heat exchanger 202.

[0057] The mixing valve 226 positioned upstream of the compressor 204 and downstream of the heat exchanger 202 receives the first BOG stream 1 that exits the heat exchanger 202 (i.e. no gas received from BOG stream 2). From the mixing valve 226, the first BOG stream 1 may be delivered to the compressor 204 for compression processing.

[0058] In both the first and second scenarios, the reliquefaction system 200 includes a heat exchanger 202 that is configured as a multi-stream heat exchanger that processes multiple gas streams simultaneously within the reliquefaction system. The heat exchanger 202 may handle four different streams simultaneously, enabling thermal exchange operations between various process00243.0032US02flows. The multi-stream configuration allows the heat exchanger 202 to serve multiple functions within the reliquefaction process while maintaining thermal efficiency across different operating conditions.

[0059] The heat exchanger 202 of system 200 may be constructed as a Brazed Aluminum Heat Exchanger (BAHX) or a Stainless Steel Welded heat exchanger. The selection of heat exchanger type may depend on operational parameters, process conditions, and system requirements. The BAHX configuration may provide thermal performance characteristics, while the Stainless Steel Welded configuration may offer operational flexibility, resistance to process fluctuations, and fast reaction time, for example the system can start and stop the required liquefaction in a shorter time than the BAHX.

[0060] The heat exchanger 202 is configured to receive and process the first BOG stream 1 as one of the multiple process streams. The first BOG stream 1 flows through the heat exchanger 202 and may undergo temperature changes during the thermal exchange process. The heat exchanger 202 increases the temperature of the first BOG stream 1 as the first BOG stream 1 provides cooling capacity to other streams within the heat exchanger 202.

[0061] A fourth BOG stream 4 is delivered to the heat exchanger 202 for reliquefaction processing. The fourth BOG stream 4 may represent compressed boil-off gas that undergoes cooling and phase change within the heat exchanger 202. The heat exchanger 202 may be configured to receive the fourth BOG stream 4 and decrease the temperature of the fourth BOG stream 4 during the reliquefaction process. The temperature reduction of the fourth BOG stream 4 may facilitate the conversion from gaseous phase to liquid phase.

[0062] A refrigeration loop 216 provides thermal interaction with the heat exchanger 202 through refrigerant streams. The heat exchanger 202 is configured to receive a pressurized refrigerant stream from the refrigeration loop 216. The refrigeration loop 216 includes a compressor-expander 206 and at least one refrigerant compressor 220. The pressurized refrigerant stream may undergo cooling within the heat exchanger 202, and the heat exchanger 202 may decrease a temperature of the pressurized refrigerant stream. In some cases, the compressed refrigerant may be cooled to a temperature range of about -90°C to -110°C within the heat exchanger 202.

[0063] The heat exchanger 202 may also be configured to receive an expanded refrigerant stream from the refrigeration loop 216. The expanded refrigerant stream may flow through the heat exchanger 202 and may undergo temperature changes during the thermal exchange process. The heat exchanger 202 may increase the temperature of the expanded refrigerant stream as the expanded refrigerant stream provides cooling capacity to other process streams.00243.0032US02

[0064] Thus, a first cooling source and a second cooling source may drive a cooling process within the heat exchanger 202 to reliquefy boil-off gas. The first cooling source is provided by the first BOG stream 1, while the second cooling source is provided by the refrigeration loop 216. The combination of the first cooling source and the second cooling source enables the heat exchanger 202 to achieve the thermal conditions for reliquefaction of the fourth BOG stream 4.

[0065] Process parameter tweaking may be implemented to create higher mean temperature difference (MTD) in a gas-gas portion of the heat exchanger 202. The process parameter adjustments may affect the thermal performance characteristics of the heat exchanger 202 and may influence the heat transfer efficiency between different process streams. In some cases, the higher MTD may be created to optimize heat exchanger size or heat transfer area for the thermal exchange operations and accommodate different heat exchanger configurations within the reliquefaction system.

[0066] The compressor 204 may be configured to compress the mixture of the first BOG stream 1 and the second BOG stream 2 that is delivered from the mixing valve 226. The compressor 204 receives the combined gas streams and applies compression processing to increase a pressure of the mixed boil-off gas. The compressor 204 may compress the BOG mixture to a pressure range of about 9 to 12 bar, which may provide the pressure conditions for subsequent processing operations within the reliquefaction system.

[0067] The compressor 204 may be configured as a positive displacement type compressor that provides compression through mechanical displacement of gas volumes. In some cases, the compressor 204 may be configured as a gas screw compressor that utilizes rotating screw elements to compress the boil-off gas mixture, such as an oil-free gas screw compressor. The gas screw configuration may provide compression characteristics that are suitable for processing boil-off gas in reliquefaction applications. Embodiments of the compressor 204 may also be a centrifugal compressor or other compressor that utilizes dynamic compression. Further embodiments include a piston compressor or any device that is capable of gas compression.

[0068] A compressor core of the compressor 204 may be the same type of compressor core as a refrigerant compressor(s) used within the refrigeration loop 216. The common compressor core design may provide operational advantages through standardized components and maintenance procedures. In some cases, the use of the same compressor core type may reduce system complexity and provide cost benefits through component commonality across different system functions.

[0069] The compressor 204 may deliver the compressed BOG mixture to a second valve 224 that is positioned downstream of the compressor 204. The second valve 224 may be located00243.0032US02upstream of the heat exchanger 202 and may control the distribution of the compressed gas mixture to different system pathways. The second valve 224 may separate the compressed mixture into multiple flow streams based on operational parameters and system demands.

[0070] The second valve 224 separates the compressed mixture into the third BOG stream 3 and the fourth BOG stream 4. The third BOG stream 3 may be fed to a gas consumer according to gas consumer load requirements, while the fourth BOG stream 4 may be delivered to the heat exchanger 202 for reliquefaction processing. The separation by the second valve 224 may be controlled to balance the gas distribution between consumption and reliquefaction functions within the system.

[0071] The third BOG stream 3 may be directed toward gas consumers that utilize boil-off gas for operational purposes. The gas consumers may include engines, generators, or other equipment that consume boil-off gas as fuel or for other operational functions. The third BOG stream 3 may provide the compressed boil-off gas at pressure and temperature conditions that are suitable for consumption by the downstream equipment.

[0072] The fourth BOG stream 4 may be delivered to the heat exchanger 202 where the fourth BOG stream 4 may undergo reliquefaction processing. The fourth BOG stream 4 may represent the portion of the compressed boil-off gas that is designated for conversion back to liquid phase. The heat exchanger 202 may process the fourth BOG stream 4 through thermal exchange operations that reduce the temperature and facilitate phase change from gas to liquid.

[0073] The compressor 204 is further configured to compress the BOG that exits the heat exchanger 202 after thermal processing. The BOG exiting the heat exchanger 202 may include the first BOG stream 1 that has been warmed during the heat exchange process. The compressor 204 may receive this warmed BOG along with the second BOG stream 2 and may apply compression to the combined streams for further processing within the reliquefaction system.

[0074] The refrigeration loop 216 may be configured to provide a second cooling source to the heat exchanger 202 through thermal exchange operations with refrigerant streams. The refrigeration loop 216 may comprise at least one refrigerant compressor 220 and at least one Expander-Compressor 206 that work together to generate cooling capacity for the reliquefaction process. The refrigeration loop 216 may operate as a closed-loop system that circulates refrigerant through compression, cooling, and expansion cycles to extract thermal energy from the boil-off gas streams.

[0075] A refrigerant compressor 220 is positioned within the refrigeration loop 216 to provide compression processing for refrigerant streams. The refrigerant compressor 220 may be configured to compress refrigerant gas and deliver pressurized refrigerant streams to the heat exchanger 202.00243.0032US02The refrigerant compressor 220 may operate at pressure levels that enable thermal exchange operations within the heat exchanger 202. In some cases, the refrigerant compressor 220 may deliver pressurized refrigerant to a pressure range of about 16 to 18 bara.

[0076] Another refrigerant compressor 221 may be positioned within the refrigeration loop 216 in a parallel configuration with the refrigerant compressor 220. The refrigerant compressor 221 may provide additional flow capacity for the refrigeration loop 216 and may operate concurrently with the refrigerant compressor 220. The parallel configuration of the refrigerant compressor 220 and the refrigerant compressor 221 may enable the refrigeration loop 216 to handle varying refrigerant flow requirements and provide operational flexibility for different reliquefaction demands.

[0077] In an exemplary embodiment, the refrigeration loop 216 may comprise a plurality of refrigerant compressors in parallel configuration to accommodate different operational scenarios. In some cases, the refrigeration loop 216 may utilize a single refrigerant compressor 220 for lower refrigerant flow requirements, while higher flow requirements may utilize multiple refrigerant compressors operating in parallel. The selection between single or multiple refrigerant compressor configurations may depend on the maximum flowrate capacity and cost effectiveness of the refrigerant compressor core designs.

[0078] An Expander-Compressor 206 is positioned within the refrigeration loop 216 to provide compression and expansion processing for pressurized refrigerant streams. The Expander-Compressor 206 receives pressurized refrigerant that has been cooled by the heat exchanger 202 and may expand the refrigerant to lower pressure conditions. The expander portion of the Expander-Compressor 206 may expand the compressed refrigerant to approximately 2 bara pressure during the expansion process.

[0079] The Expander-Compressor 206 may be configured as a centrifugal type compressor that utilizes rotating impeller elements to process refrigerant streams. In some cases, the Expander-Compressor 206 may be configured as a positive displacement type compressor that provides expansion through mechanical displacement of refrigerant volumes. The Expander-Compressor 206 may operate without external motor drive and may not require external power for compression operations.

[0080] The refrigeration loop 216 may comprise a plurality of Expander-Compressors and at least one refrigerant compressor 220 to provide enhanced cooling capacity. The multiple Expander-Compressor 206 configuration may enable the refrigeration loop 216 to handle varying flow requirements and / or provide operational redundancy for the reliquefaction system. The00243.0032US02combination of multiple Expander-Compressors with refrigerant compressors may optimize the thermal performance of the refrigeration loop 216.

[0081] The refrigerant compressor 220 may deliver a pressurized refrigerant stream to the heat exchanger 202 where the pressurized refrigerant stream may undergo cooling through thermal exchange with other process streams. The pressurized refrigerant stream may be cooled by the heat exchanger 202 to temperature ranges that enable effective thermal exchange operations. The cooled pressurized refrigerant stream may then be expanded by the Expander-Compressor 206 to provide the second cooling source to the heat exchanger 202.

[0082] The refrigeration loop 216 may be comprised of methane gas as a refrigerant medium that circulates through the compression, cooling, and expansion cycles. In some cases, the refrigeration loop 216 may comprise methane gas, BOG, or a mixture of methane gas and nitrogen gas as refrigerant media. The use of methane gas or methane gas combined with nitrogen may provide thermal properties that are suitable for reliquefaction operations while eliminating the requirement for additional refrigerant inventory.

[0083] The refrigeration loop 216 may include automatic refrigerant replenishment capabilities that utilize fresh BOG when nitrogen blocking gas contaminates the refrigeration loop 216. The refrigerant compressors may use nitrogen as blocking gas in gas-seal solutions to prevent refrigerant leakage. In some cases, the nitrogen blocking gas may contaminate the refrigeration loop 216, and the automatic replenishment system may refresh the refrigerant charge with fresh BOG at regular intervals; the replenishment operation may also regulate a mass flow and refrigerant pressure in the loop 216.

[0084] The refrigeration loop 216 may include turndown capability that enables operation at lower refrigerant mass flowrates for partial reliquefaction applications. The turndown capability may allow the refrigeration loop 216 to operate at multiple configurations with refrigerant flowrates below design point conditions. In some cases, the refrigeration loop 216 may be configured with elevated refrigeration loop discharge pressure and BOG pressure for operational flexibility.

[0085] The liquefied gas exiting the heat exchanger 202 is processed through expansion and collection operations before being returned to the LNG tank 208. A JT valve 212 may be configured to expand the liquefied gas that exits the heat exchanger 202 after reliquefaction processing. The JT valve 212 may provide Joule- Thomson expansion that reduces the pressure of the liquefied gas while maintaining the liquid phase characteristics. Maintaining liquid phase characteristic for all or at least major portion of mass flow (after JT at least some flash gas may be expected which depends upon liquid temperature upstream of JT). The JT valve 212 may expand00243.0032US02the liquefied gas from the heat exchanger 202 operating pressure to lower pressure conditions that are suitable for collection and storage operations.

[0086] A flash drum 210 may be positioned downstream of the JT valve 212 to receive the expanded liquefied gas from the expansion process. The flash drum 210 may be configured for receiving the liquefied gas that has undergone pressure reduction through the JT valve 212. The flash drum 210 may operate at approximately 4.5 bara pressure, which may provide pressure conditions that enable separation of liquid and vapor phases within the flash drum 210. The flash drum 210 may collect the liquefied gas and may facilitate the separation of any vapor components that may form during the expansion process.

[0087] The flash drum 210 can enable the return of liquefied gas to the LNG tank 208 through pressure- driven flow operations. The liquefied gas from the flash drum 210 may be at pressure levels that are sufficient to enable flow below the liquid level within the LNG tank 208. The flash drum 210 may maintain pressure conditions that allow the liquefied gas to be transferred back to the LNG tank 208 without additional pumping operations.

[0088] A gas combustion unit 214 can be positioned to receive vent gas that exits the flash drum 210 during the liquid collection process. The gas combustion unit 214 may be configured to combust small quantities of flash gas that are separated from the liquefied gas within the flash drum 210. The vent gas from the flash drum 210 may contain high concentrations of noncondensable components that are removed from the reliquefaction process to maintain system efficiency.

[0089] The vent gas from the flash drum 210 may contain nitrogen concentrations of 50% or more by molecular weight. The high nitrogen content in the vent gas may result from the accumulation of non-condensable components during the reliquefaction process. The gas combustion unit 214 may combust this nitrogen-rich vent gas to prevent the accumulation of non-condensable components within the reliquefaction system. The flash gas flow rate may be approximately 50 kg / h during normal operation conditions.

[0090] A mass flow rate measurement device may be positioned in fluid communication with the flash drum 210 to monitor the mass flow rate of the vent gas exiting the flash drum 210. The mass flow rate measurement device may provide continuous monitoring of the vent gas flow characteristics and may enable operational control based on vent gas flow parameters. In some cases, when the mass flow rate of the vent gas exceeds a predetermined threshold, the operation of the compressor 204 may be changed to adjust the reliquefaction process parameters.

[0091] The mass flow rate measurement device may enable system control through monitoring of the vent gas flow rate from the flash drum 210. The vent gas flow rate may serve as00243.0032US02an indication of the working condition of the reliquefaction system and may be used as a system control parameter. In some cases, when the vent gas flow rate exceeds predetermined limits, the compressor 204 speed may be reduced to extract less BOG from the LNG tank 208, thereby adjusting the reliquefaction system operation to maintain optimal performance conditions.

[0092] Turning now to embodiments of a reliquefaction method 300 using system 200. FIG.3 illustrates a flowchart for a reliquefaction method 300 that processes boil-off gas through a dual cooling source system according to a first scenario. In the first scenario, a gas consumer load requirement exceeds a minimum threshold, and the BOG extracted from the LNG tank 208 is separated into a first BOG stream 1 and a second BOG stream 2.

[0093] The method 300 begins at step 302 with the extraction of BOG from an LNG tank 208. The extraction process may involve drawing the gaseous phase from the tank's vapor space to maintain pressure control while providing boil-off gas for reliquefaction processing. In the reliquefaction system 200, this extraction is performed by the compressor 204, which draws the BOG stream 218 from the LNG tank 208 through controlled flow operations that establish the initial conditions for the reliquefaction process.

[0094] The method 300 proceeds to step 304 where a gas consumer load requirement is determined. The gas consumer load requirement may represent the demand for boil-off gas from engines, generators, or other equipment that utilize the gas for operational purposes. The determination of load requirements may enable the system to balance between gas consumption and reliquefaction processing based on operational demands. In system 200, this determination influences the operation of the first valve 222 and second valve 224, which control the distribution of gas streams throughout the system based on real-time consumer demand assessments.

[0095] At step 306, the method 300 includes a decision point where the system evaluates whether the gas consumer load requirement exceeds a minimum threshold. This decision may determine the operational mode of the reliquefaction system and may influence the distribution of boil-off gas between consumption and reliquefaction pathways. The minimum threshold may be established based on system design parameters and operational efficiency considerations. The control logic for this decision point in system 200 is implemented through the valve control mechanisms and / or one or more processors of a computer system that governs the entire system 200 including the first valve 222 and / or the second valve 224, enabling the system to automatically adjust flow distribution based on predetermined operational criteria.

[0096] If the gas consumer load requirement exceeds the minimum threshold, the method 300 follows the "Yes" branch to step 308, where the BOG is separated into first and second streams according to the gas consumer load requirement. The separation may be controlled to allocate00243.0032US02appropriate gas quantities to different processing pathways within the reliquefaction system. The first stream may be directed toward heat exchange operations, while the second stream may be routed directly for compression processing. In system 200, this separation is accomplished by the first valve 222, which divides the BOG stream 218 into the first BOG stream 1 and the second BOG stream 2, with the first BOG stream 1 flowing to the heat exchanger 202 and the second BOG stream 2 flowing directly to the mixing valve 226.

[0097] Following step 308, the method 300 continues to step 310 where the first stream is provided to a heat exchanger 202 to serve as a first cooling source. The first stream may flow through the heat exchanger 202 and may undergo thermal processing that increases its temperature while providing cooling capacity to other process streams within the heat exchanger 202. In system 200, the heat exchanger 202 serves as a multi-stream heat exchanger that processes the first BOG stream 1 along with the fourth BOG stream 4 and refrigerant streams from the refrigeration loop 216, enabling simultaneous thermal exchange operations between multiple process flows at different temperature and pressure conditions.

[0098] The method 300 then advances to step 312 where the first and second BOG streams are mixed to form a combined BOG gas mixture. The mixing process may occur after the first stream exits the heat exchanger 202, and the resulting mixture may have thermal characteristics that are suitable for compression processing. The temperature of the mixture may be lower than the temperature of the first stream downstream of the heat exchanger 202. In system 200, this mixing operation is performed by the mixing valve 226, which combines the warmed first BOG stream 1 exiting the heat exchanger 202 with the second BOG stream 2 that has bypassed the heat exchanger 202, creating a temperature- optimized mixture that enhances compression efficiency in the compressor 204.

[0099] At step 314, the method 300 includes compression of the mixed BOG streams. The compression process may increase the pressure of the combined gas mixture to levels that enable subsequent processing operations within the reliquefaction system. The compression may be performed by a compressor 204 that processes the mixed BOG streams to achieve the desired pressure conditions. In system 200, the compressor 204 receives the combined mixture from the mixing valve 226 and compresses it to a pressure range of 9 to 12 bara, utilizing a compressor core that may be the same type as the refrigerant compressor 220 and refrigerant compressor 221 in the refrigeration loop 216 to provide operational standardization and maintenance benefits.

[0100] The method 300 proceeds to step 316 where the compressed BOG mixture is separated into third and fourth streams. The third stream may be directed to gas consumers to satisfy the gas consumer load requirement, while the fourth stream may be delivered to the heat exchanger 20200243.0032US02for reliquefaction processing. The separation may be controlled to balance the gas distribution between consumption and reliquefaction functions. In system 200, this separation is accomplished by the second valve 224, which receives the compressed BOG gas from the compressor 204 and divides it into the third BOG stream 3 for consumer supply and the fourth BOG stream 4 for reliquefaction in the heat exchanger 202, with flow rates optimized to minimize pressure drop and maximize system efficiency, which can be measured as kWh / kg of liquid formation.

[0101] The method 300 proceeds to step 320, where the fourth stream is reliquefied using dual cooling sources. The reliquefaction process may utilize the first BOG stream 1 as a first cooling source and a refrigeration loop 216 as a second cooling source to achieve the thermal conditions for converting the fourth BOG stream 4 from gaseous to liquid phase. The dual cooling mechanism may enable effective reliquefaction while maintaining operational flexibility for different consumer load scenarios. In system 200, this reliquefaction occurs within the heat exchanger 202, where the first BOG stream 1 provides the first cooling source, and the refrigeration loop 216 comprising the refrigerant compressor 220 and the Expander-Compressor 206, provides the second cooling source. In some embodiments, the refrigeration loop also includes additional refrigerant compressors, such as refrigerant compressor 221. The reliquefied gas then flows through the JT valve 212 to the flash drum 210, where it is collected and returned to the LNGtank 208, while any vent gas is processed by the gas combustion unit 214.

[0102] If the gas consumer load requirement does not exceed the minimum threshold at step 306, the method 300 follows the "No" branch to step 318, where the BOG is processed according to a second scenario, in which the extracted BOG 218 bypasses certain separation and mixing operations. The second mode of operation may direct the boil-off gas to the heat exchanger 202 to increase the cooling capacity of the first cooling source. The second operational mode may involve different valve positions in the first valve 222 and second valve 226, allowing for full reliquefaction operation where the majority or all of the compressed BOG is processed through the heat exchanger 202 without significant diversion to consumer supply.

[0103] FIG. 4 illustrates a flowchart for a reliquefaction method 500 that processes boil-off gas through a dual cooling source system according to a second scenario. In a second scenario, a gas consumer load requirement is at or below a minimum threshold, and the BOG 218 exiting the LNG tank 208 is sent only as a first BOG stream 1 to maximize the first cooling source for the heat exchanger 202.

[0104] The method 500 begins at step 502 with the extraction of BOG 218 from an LNG tank 208. The extraction process may involve drawing the gaseous phase from the tank's vapor space to maintain pressure control while providing boil-off gas for reliquefaction processing. In the00243.0032US02reliquefaction system 200, this extraction is performed by the compressor 204, which draws the BOG stream 218 from the LNG tank 208 through controlled flow operations that establish the initial conditions for the reliquefaction process.

[0105] The method 500 proceeds to step 504 where a gas consumer load requirement is determined. The gas consumer load requirement may represent the demand for boil-off gas from engines, generators, or other equipment that utilize the gas for operational purposes. The determination of load requirements may enable the system to balance between gas consumption and reliquefaction processing based on operational demands. In system 200, this determination influences the operation of the first valve 222 and second valve 224, which control the distribution of gas streams throughout the system based on real-time consumer demand assessments.

[0106] At step 506, the method 500 includes a decision point where the system evaluates whether the gas consumer load requirement exceeds a minimum threshold. This decision may determine the operational mode of the reliquefaction system and may influence the distribution of boil-off gas between consumption and reliquefaction pathways. The minimum threshold may be established based on system design parameters and operational efficiency considerations. The control logic for this decision point in system 200 is implemented through the valve control mechanisms and / or one or more processors of a computer system that govern the entire system 200 including the first valve 222 and / or the second valve 224, enabling the system to automatically adjust flow distribution based on predetermined operational criteria.

[0107] If the gas consumer load requirement is at or below the minimum threshold, the method 500 follows the "Yes" branch to step 508, where the BOG stream 218 is sent to the heat exchanger 202 for maximum cooling capacity as the first cooling source; no portion of the BOG 218 bypasses the heat exchanger 202. The first BOG stream 1 may flow through the heat exchanger 202 and may undergo thermal processing that increases its temperature while providing cooling capacity to other process streams within the heat exchanger 202. In system 200, the heat exchanger 202 serves as a multi-stream heat exchanger that processes the first BOG stream 1 along with a reliquefaction stream (labeled as the fourth BOG stream 4 in FIG. 2) and refrigerant streams from the refrigeration loop 216, enabling simultaneous thermal exchange operations between multiple process flows at different temperature and pressure conditions.

[0108] The method 500 then advances to step 512 where the first BOG stream 1 exits the heat exchanger 202 and flows to the compressor 204.

[0109] At step 514, the method 500 includes compression of the BOG stream exiting the heat exchanger 202 by the compressor 204. The compression process may increase the pressure of the gas to levels that enable subsequent processing operations within the reliquefaction system. The00243.0032US02compression may be performed by a compressor 204 that processes the BOG stream to achieve the desired pressure conditions. In system 200, the compressor 204 receives the BOG stream from the heat exchanger 202 and compresses it to a pressure range of about 9 to 12 bara, utilizing a compressor core that may be the same type as the refrigerant compressor 220 and refrigerant compressor 221 core in the refrigeration loop 216 to provide operational standardization and maintenance benefits.

[0110] The method 500 proceeds to step 516 where the compressed BOG is separated into third and fourth streams 3, 4. The third stream 3 may be directed to gas consumers to satisfy the gas consumer load requirement, while the fourth stream 4 may be delivered to the heat exchanger 202 for reliquefaction processing. The separation may be controlled to balance the gas distribution between consumption and reliquefaction functions. In system 200, this separation is accomplished by the second valve 224, which receives the compressed BOG gas from the compressor 204 and divides it into the third BOG stream 3 for consumer supply and the fourth BOG stream 4 for reliquefaction in the heat exchanger 202.

[0111] The method 500 proceeds to step 518, where the fourth stream is reliquefied using dual cooling source. The reliquefaction process may utilize the first BOG stream 1 as a first cooling source and a refrigeration loop 216 as a second cooling source to achieve the thermal conditions for converting the fourth BOG stream 4 from gaseous to liquid phase. The dual cooling mechanism may enable effective reliquefaction while maintaining operational flexibility for different consumer load scenarios. In system 200, this reliquefaction occurs within the heat exchanger 202, where the first BOG stream 1 provides the first cooling source and the refrigeration loop 216, comprising the refrigerant compressor 220, and / or refrigerant compressor 221, and Expander-Compressor 206, provides the second cooling source. The reliquefied gas then flows through the JT valve 212 to the flash drum 210, where it is collected and returned to the LNG tank 208, while any flash / vent gas is processed by the gas combustion unit 214.

[0112] If the gas consumer load requirement does exceed the minimum threshold at step 506, the method 500 follows the "No" branch to step 520, where the BOG is processed according to the first scenario, in which the extracted BOG 218 is separated into at least two streams upstream of the heat exchanger 202.

[0113] FIG. 5 illustrates a flowchart for a refrigeration loop 216 operation method 600 that demonstrates the operational control of the refrigeration loop 216 within the reliquefaction system 200. The method 600 provides a systematic approach for managing the refrigeration components that supply the second cooling source to the heat exchanger 202.00243.0032US02

[0114] The method 600 begins at step 602 with refrigeration loop 216 operation startup, where the refrigeration loop 216 components are initialized for operation. The startup process may involve preparing the refrigerant compressor 220, refrigerant compressor 221, and Expander-Compressor 206 for coordinated operation within the refrigeration loop 216. The initialization may establish the operational parameters for methane gas, BOG gas, and / or a mixture of methane and nitrogen gas circulation through the refrigeration cycle.

[0115] The method 600 proceeds to step 604, which includes a decision point where the system determines whether multiple compressors are needed based on refrigerant flow requirements and system demand. This decision may be derived from cooling capacity requirement (e.g. the heat exchanger 202 design may be for the worst case and may have no bearing on the decision as to what refrigerant flowrate may be required for the partial reliquefaction scenario), and the operational efficiency considerations for the refrigeration loop 216.

[0116] If multiple compressors are needed, the method 600 follows the "Yes" branch to step 606, where parallel refrigerant compressors are operated. In system 200, this operation may involve concurrent operation of both the refrigerant compressor 220 and the refrigerant compressor 221 in parallel configuration to provide enhanced flow capacity for the refrigeration loop 216. The parallel operation may enable the system to handle higher refrigerant flow rates and provide increased cooling capacity.

[0117] If multiple compressors are not needed, the method 600 follows the "No" branch to step 608, where a single refrigerant compressor 220 is operated. In system 200, this operation may involve utilizing either the refrigerant compressor 220 or the refrigerant compressor 221 individually to provide compression for the refrigeration loop 216.

[0118] Both operational pathways converge at step 610, where pressurized refrigerant is delivered to the heat exchanger 202. In system 200, the pressurized refrigerant stream from either the single or parallel compressor configuration flows to the heat exchanger 202 where the refrigerant undergoes thermal exchange with other process streams. The pressurized refrigerant may be delivered at pressure levels of about 16 to 18 bara to enable effective thermal exchange operations.

[0119] The method 600 continues to step 612, where the pressurized refrigerant is cooled in the heat exchanger 202. In system 200, the heat exchanger 202 processes the pressurized refrigerant stream and reduces its temperature through thermal exchange with other process streams. The cooling process may reduce the refrigerant temperature to a range of about -90°C to -110°C, preparing the refrigerant for expansion processing.00243.0032US02

[0120] Following the cooling process, the method 600 proceeds to step 614, where the cooled refrigerant is expanded through the expander part of the Expander-Compressor 206. In system 200, the Expander-Compressor 206 receives the cooled pressurized refrigerant from the heat exchanger 202 and expands the refrigerant to lower pressure conditions of approximately 2 bara. The expansion process may generate the cooling effect that provides the second cooling source for the reliquefaction operations.

[0121] The method 600 concludes at step 616, where the second cooling source is provided to the heat exchanger 202. In system 200, the expanded refrigerant from the Expander-Compressor 206 flows through the heat exchanger 202 and absorbs thermal energy from the fourth BOG stream 4 during the reliquefaction process, and can remove heat from the compressed refrigerant stream. The expanded refrigerant may provide cooling capacity that works in conjunction with the first cooling source from the first BOG stream 1 to achieve effective reliquefaction of the fourth BOG stream 4 within the heat exchanger 202.

[0122] FIG. 6 illustrates a composite temperature profile graph that demonstrates the thermal performance characteristics of a BAHX heat exchanger 202. The graph displays the relationship between heat flow and temperature for the cold and hot composite streams within the multi-stream heat exchanger 202 during reliquefaction operations.

[0123] The horizontal axis represents heat flow measured in kilowatts, while the vertical axis shows temperature measured in degrees Celsius. The graph presents two distinct curves that represent the thermal behavior of different process streams within the heat exchanger 202.

[0124] The cold composite curve begins at approximately -150°C at zero heat flow and rises steadily to reach approximately 40°C at maximum heat flow. This curve may represent the thermal profile of the process streams that provide cooling capacity within the heat exchanger 202, including the first BOG stream 1 and the expanded refrigerant from the Expander-Compressor 206. The cold composite curve demonstrates how these cooling sources absorb thermal energy as they progress through the heat exchanger 202.

[0125] The hot composite curve starts at approximately 40 °C and follows the shown trajectory until about -150 °C. This curve may represent the thermal profile of the process streams that require cooling within the heat exchanger 202, including the fourth BOG stream 4 that undergoes reliquefaction and the pressurized refrigerant from the refrigerant compressor 220 and refrigerant compressor 221.

[0126] Both curves exhibit a generally linear relationship within gaseous phases between heat flow and temperature (e.g. for most part of the heat exchange processes, especially in the gas-gas heat exchange sections), indicating consistent thermal exchange characteristics throughout the00243.0032US02operating range of the heat exchanger 202. The hot composite curve is positioned above the cold composite curve throughout the operating range, creating the temperature differential that drives the heat transfer process within the reliquefaction system.

[0127] A small mean temperature difference is indicated between the two curves in the region around where the fourth stream 4 starts to liquefy. This small temperature differential represents the pinch point within the heat exchanger 202, which dictates the heat exchanger process.

[0128] The temperature profile visualization in FIG. 6 may provide insights into the thermal performance of the heat exchanger 202 and may assist in optimizing the reliquefaction system design. The composite curves demonstrate how the dual cooling sources work together to achieve the thermal conditions for effective BOG reliquefaction while maintaining operational efficiency across different heat flow conditions.

[0129] FIG. 7 illustrates an alternative composite temperature profile graph that demonstrates typical thermal performance characteristics of a Stainless Steel Welded heat exchanger 202 under modified operational conditions. The graph displays the relationship between heat flow and temperature for the cold and hot composite streams within the multi-stream heat exchanger 202 during optimized reliquefaction operations.

[0130] The horizontal axis represents heat flow measured in kilowatts, while the vertical axis shows temperature measured in degrees Celsius. The graph presents two distinct curves that represent the thermal behavior of different process streams within the heat exchanger 202 under tweaked operational parameters.

[0131] The cold composite curve begins at approximately -150°C at zero heat flow and rises steadily to reach approximately 20°C at maximum heat flow. This curve may represent the thermal profile of the process streams that provide cooling capacity within the heat exchanger 202, including the first BOG stream 1 and the expanded refrigerant from the Expander-Compressor 206 under tweaked process conditions. The cold composite curve demonstrates how these cooling sources absorb thermal energy as they progress through the heat exchanger 202 with tweaked thermal exchange characteristics.

[0132] The hot composite curve starts at approximately 40 °C and follows a similar trajectory until about 150 °C. This curve may represent the thermal profile of the process streams that require cooling within the heat exchanger 202, including the fourth BOG stream 4 that undergoes reliquefaction and the pressurized refrigerant from the refrigerant compressor 220 and refrigerant compressor 221 under tweaked operational parameters.00243.0032US02

[0133] Both curves exhibit a generally linear relationship within gaseous phases between heat flow and temperature (e.g. or most part of the heat exchange process especially in the gas-gas heat exchange sections), indicating consistent thermal exchange characteristics throughout the extended operating range of the heat exchanger 202. The positioning of the curves creates an enhanced temperature differential that drives the heat transfer process within the reliquefaction system with improved efficiency characteristics.

[0134] A larger mean temperature difference is indicated between the two curves in a region of gas-to-gas heat exchanger sections. This larger temperature differential represents an improved thermal driving force within the heat exchanger 202, where the enhanced temperature difference may enable more effective heat transfer operations. The larger mean temperature difference may result from process parameter tweaking that creates higher mean temperature difference in the gasgas portion of the heat exchanger 202, potentially enabling heat exchanger size (or heat exchange) area reduction. The higher MTD also means that B AHX may not be able to take higher temperature differences, potentially necessitating use of Stainless Steel Welded heat exchanger, which also allows for greater operational flexibility.

[0135] The temperature profile visualization in FIG. 7 may provide insights into the enhanced thermal performance of the heat exchanger 202 under tweaked operational conditions and may assist in evaluating alternative system configurations. The composite curves demonstrate how process parameter adjustments may improve the thermal conditions for effective BOG reliquefaction while maintaining operational efficiency across extended heat flow ranges and enabling greater operational flexibility.

[0136] Embodiments of the methods 300, 500, 600 can be computer implemented. For instance, the reliquefaction system 200 may include one or more processors of a computer system that enable automated control and monitoring of the reliquefaction process. The processors may be configured to execute computer-readable instructions stored in memory to implement the reliquefaction methods described herein. The computer system may include processing units, memory modules, input / output interfaces, and communication components that facilitate system operation and control.

[0137] The one or more processors may be configured to monitor operational parameters throughout the reliquefaction system 200, including pressure levels, temperature measurements, flow rates, and valve positions. The processors may receive sensor data from various components within the system, such as pressure sensors positioned at the LNG tank 208, heat exchanger 202, compressor 204, and flash drum 210. Temperature sensors may provide thermal data from multiple00243.0032US02locations within the heat exchanger 202 and refrigeration loop 216, downstream of valve 212 to enable real-time monitoring of the reliquefaction process.

[0138] The computer system may include one or more controllers that manage the operation of individual system components. A compressor controller may regulate the operation of the compressor 204, adjusting compression parameters based on BOG flow requirements and liquefaction demand. The compressor controller may modify compressor speed, and discharge pressure to optimize reliquefaction performance while maintaining operational efficiency.

[0139] A valve controller may manage the positioning and operation of the first valve 222, second valve 224, based on gas consumer load requirements and system operational parameters. The valve controller may receive input signals from the processors regarding consumer demand and may adjust valve positions to direct appropriate gas flow quantities to different system pathways. The valve controller may also coordinate valve operations to maintain optimal pressure drop characteristics within the heat exchanger 202.

[0140] A refrigeration loop controller may manage the operation of the refrigeration loop 216, including the refrigerant compressor 220, refrigerant compressor 221, and Expander-Compressor 206. The refrigeration loop controller may determine whether reliquefaction needs to operate with single or parallel compressor operation based on cooling capacity requirements, and may adjust refrigerant flow rates to provide the second cooling source to the heat exchanger 202. The controller may monitor refrigerant pressure levels and temperature conditions to optimize the thermal exchange operations within the heat exchanger 202.

[0141] The one or more processors may implement control algorithms that coordinate the operation of multiple system components to achieve effective BOG reliquefaction. The control algorithms may include decision logic for determining gas consumer load requirements, flow distribution strategies, and operational mode selection based on real-time system conditions. The processors may execute feedback control loops that adjust system parameters in response to changing operational demands and performance metrics.

[0142] The computer system may include a mass flow rate monitoring module that processes signals from the mass flow rate measurement device positioned in fluid communication with the flash drum 210. The monitoring module may track the mass flow rate of vent gas exiting the flash drum 210 and may trigger operational adjustments when the vent gas flow rate exceeds predetermined thresholds. In response to elevated vent gas flow rates, the processors may reduce compressor 204 speed to extract less BOG from the LNG tank 208, thereby maintaining optimal reliquefaction system performance, at the same time giving time for an operation crew to monitor proper functioning of the reliquefaction system and take corrective actions if needed.00243.0032US02

[0143] The processors may implement safety monitoring functions that continuously evaluate system operational parameters against predetermined safety limits. The safety monitoring may include pressure monitoring, temperature monitoring, and flow rate monitoring across multiple system components. The processors may initiate protective actions, such as system shutdown or operational mode changes, when safety parameters are exceeded or when equipment malfunctions are detected.

[0144] The computer system may include data logging capabilities that record operational parameters, performance metrics, and system events for analysis and optimization purposes. The data logging may capture time-stamped information regarding BOG flow rates, reliquefaction efficiency, energy consumption, and equipment performance. The logged data may be used to validate component performance against the component’s design, optimize system operation, predict maintenance requirements, and improve overall reliquefaction system performance.

[0145] The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

[0146] The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension or values is intended to mean both the recited value and a functionally equivalent range surrounding that value.

[0147] A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.

Claims

00243.0032US02CLAIMS1. A reliquefaction system comprising:a compressor configured to:extract a Boil-Off Gas (BOG) from a Liquefied Natural Gas (LNG) tank and provide at least a portion of the BOG to a heat exchanger to be used as a first cooling source; compress the BOG exiting the heat exchanger, anddeliver the compressed BOG to be reliquefied to the heat exchanger; anda refrigeration loop configured to provide a second cooling source to the heat exchanger; wherein the first cooling source and the second cooling source drive a cooling process in the heat exchanger to reliquefy the BOG.

2. The reliquefaction system of claim 1, further comprising:a first valve upstream of the compressor and the heat exchanger, which separates the BOG into a first BOG stream and a second BOG stream based on a gas consumer load requirement, wherein the first BOG stream is provided to the heat exchanger as the first cooling source and the second BOG stream is provided to the compressor to form a mixture with the BOG exiting the heat exchanger; anda second valve downstream of the compressor and upstream of the heat exchanger, which separates the mixture into a third BOG stream fed to a gas consumer according to the gas consumer load requirement, and a fourth BOG stream to be delivered to the heat exchanger for reliquefaction.

3. The reliquefaction system of claim 2, wherein the first BOG stream is heated by the heat exchanger and then mixed with the second BOG stream to form the mixture, which has a temperature lower than the first BOG stream downstream of the heat exchanger.00243.0032US024. The reliquefaction system of claim 3, wherein a flowrate of the second BOG stream is derived from a flowrate of the third BOG stream to optimize a pressure drop in the heat exchanger.

5. The reliquefaction system of claim 1, wherein all of the extracted BOG is provided to the heat exchanger to maximize a cooling capacity of the first cooling source.

6. The reliquefaction system of claim 1, wherein the refrigeration loop comprises: at least one refrigerant compressor and at least one Expander-Compressor, a plurality of refrigerant compressors in parallel and the at least one Expander-Compressor, or a plurality of Expander-Compressors in parallel and at least one refrigerant compressor.

7. The reliquefaction system of claim 1, wherein the at least one refrigerant compressor of the refrigeration loop delivers a pressurized refrigerant stream to the heat exchanger, the pressurized refrigerant stream being cooled by the heat exchanger and then expanded by the at least one Expander-Compressor to provide the second cooling source to the heat exchanger.

8. The reliquefaction system of claim 7, wherein a compressor core of the compressor is a same type of compressor core as the at least one refrigerant compressor.

9. The reliquefaction system of claim 1, further comprising:a Joules Thompson (JT) valve configured to expand a liquefied gas exiting the heat exchanger, and a flash drum for receiving the liquefied gas, wherein the liquefied gas is returned to the LNG tank from the flash drum; anda mass flow rate measurement device in fluid communication with the flash drum to monitor a mass flow rate of a vent gas exiting the flash drum, wherein, in response to the mass00243.0032US02flow rate of the vent gas exceeding a predetermined threshold, an operation of the compressor is changed and / or operational parameters of the refrigeration loop are changed.

10. The reliquefaction system of claim 1, wherein the heat exchanger is a Brazed Aluminum Heat Exchanger (BAHX) or a Stainless Steel Welded Heat Exchanger.

11. The reliquefaction system of claim 2, wherein the heat exchanger is a multi-stream heat exchanger, and is configured to:receive the first BOG stream and increase a temperature of the first BOG stream; receive the fourth BOG stream and decrease a temperature of the fourth BOG stream; receive a pressurized refrigerant stream from at least one refrigerant compressor of the refrigeration loop and decrease a temperature of the pressurized refrigerant stream; andreceive an expanded refrigerant stream from an Expander-Compressor of the refrigeration loop and increase a temperature of the expanded refrigerant stream.

12. The reliquefaction system of claim 1, wherein the heat exchanger is a multi-stream heat exchanger, and is configured to:receive the BOG and increase a temperature of the BOG;receive a reliquefaction stream from the compressor and decrease a temperature of the reliquefaction stream;receive a pressurized refrigerant stream from at least one refrigerant compressor of the refrigeration loop and decrease a temperature of the pressurized refrigerant stream; andreceive an expanded refrigerant stream from an Expander-Compressor of the refrigeration loop and increase a temperature of the expanded refrigerant stream.00243.0032US0213. A reliquefaction method comprising:extracting a Boil-Off Gas (BOG) from a Liquefied Natural Gas (LNG) tank; determining a gas consumer load requirement;in response to the gas consumer load requirement exceeding a minimum threshold, separating the BOG according to the gas consumer load requirement into a first BOG stream that flows to a heat exchanger to be used as a first cooling source and a second BOG stream that flows directly to a compressor;mixing the first BOG stream and the second BOG stream to form a mixture of the first BOG stream and the second BOG stream that is compressed by the compressor; separating the compressed mixture into a third BOG stream to be delivered to a gas consumer and a fourth BOG stream to be delivered to the heat exchanger for reliquefaction; andreliquefying the fourth BOG stream using the first cooling source and a second cooling source provided by a refrigeration loop.

14. The reliquefaction method of claim 13, further comprising: determining a flowrate of the second BOG stream based on a flowrate of the third BOG stream to optimize a pressure drop in the heat exchanger and improving compressor Efficiency of a BOG compressor.

15. The reliquefaction method of claim 13, wherein the refrigeration loop comprises at least one refrigerant compressor and at least one Expander-Compressor.

16. The reliquefaction method of claim 15, further comprising: expanding, by the Expander Compressor, a pressurized refrigerant stream pressurized by the at least one refrigerant compressor and cooled by the heat exchanger to deliver the second cooling source to the heat exchanger.

17. The reliquefaction method of claim 16, further comprising:00243.0032US02expanding a liquefied gas exiting the heat exchanger using a Joules Thompson (JT) valve and collecting the liquefied gas in a flash drum; andreturning the liquefied gas to the LNGtank from the flash drum.

18. The reliquefaction method of claim 13, wherein the heat exchanger is a multi-stream heat exchanger, and is configured to:receive the first BOG stream and increase a temperature of the first BOG stream; receive the fourth BOG stream and decrease a temperature of the fourth BOG stream; receive a pressurized refrigerant stream from at least one refrigerant compressor of the refrigeration loop and decrease a temperature of the pressurized refrigerant stream; andreceive an expanded refrigerant stream from an Expander-Compressor of the refrigeration loop and increase a temperature of the expanded refrigerant stream.

19. A reliquefaction method comprising:extracting BOG from a LNG tank;determining that a gas consumer load requirement is at or below a gas consumer load requirement minimum threshold;sending the BOG to a heat exchanger to be used as a first cooling source, which flows to a compressor downstream of the heat exchanger that compresses the gas exiting the heat exchanger;separating the compressed gas into a consumer stream to be delivered to a gas consumer and a reliquefaction stream to be delivered to the heat exchanger for reliquefaction; and reliquefying the reliquefaction stream using the first cooling source and a second cooling source provided by a refrigeration loop.00243.0032US0220. The reliquefaction method of claim 19, wherein all of the BOG extracted from the LNG tank is sent to the heat exchanger to maximize a cooling capacity of the first cooling source.

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