Ship's evaporative gas reliquefaction system and method, and ship's reliquefaction device off-gas treatment system and method

Abstract

The present invention discloses a system and method for reliquefying evaporated gas on a ship, as well as a system and method for treating off-gas from a reliquefaction device for a ship. The system for reliquefying evaporated gas on a ship includes a compressor that compresses evaporated gas generated from liquefied gas stored in a storage tank on the ship, a heat exchanger that cools the compressed gas compressed by the compressor, a refrigerant circulation line that circulates refrigerant supplied to the heat exchanger, a heating line that connects the storage tank and the compressor, and a heater provided in the heating line, and the heater heats the evaporated gas to the appropriate intake temperature required by the compressor. [Figure 2]

Description

[Technical field]

[0001] The present invention relates to a reliquefaction system and a method for reliquefying boil-off gas (BOG) generated from liquefied gas stored in a storage tank of a ship and recovering it in a storage tank, as well as an off-gas treatment system and an off-gas treatment method for a reliquefaction device that can discharge and treat off-gas with a high nitrogen content separated in a separator of the reliquefaction device, thereby maintaining the reliquefaction performance of the reliquefaction device. [Background technology]

[0002] Natural gas is primarily composed of methane and emits almost no environmental pollutants when burned, so it has been drawing attention as an environmentally friendly fuel. Liquefied natural gas (LNG) is obtained by liquefying natural gas by cooling it to approximately -163°C at atmospheric pressure. Since the volume of liquefied natural gas is reduced to approximately 1 / 600 of that of gaseous natural gas, it is highly suitable for long-distance transportation via sea routes. Therefore, natural gas is mainly stored and transported in the liquefied natural gas state, which is easy to store and transport.

[0003] Since the liquefaction point of natural gas is an extremely low temperature of approximately -163°C at atmospheric pressure, LNG storage tanks are generally insulated to maintain LNG in a liquid state. However, even if LNG storage tanks are insulated, there is a limit to how much they can block external heat, and as external heat continues to be transferred to the LNG storage tank, the LNG continues to naturally evaporate inside the LNG storage tank during the LNG transportation process, generating evaporated gas.

[0004] The internal pressure of an LNG storage tank increases as evaporation gas continues to be generated inside the tank. If the internal pressure of an LNG storage tank exceeds the set safety pressure, it may cause an emergency such as tank rupture, so it is necessary to use a safety valve to release the evaporation gas to the outside of the storage tank. However, since evaporation gas is one of the LNG losses and is a major problem in terms of LNG transportation efficiency and fuel efficiency, various methods are used to treat the evaporation gas generated in storage tanks.

[0005] In recent years, methods have been developed and are being used that involve using evaporative gas at fuel demand sources such as ship engines, re-liquefying evaporative gas and recovering it in storage tanks, or a combination of these two methods. Summary of the Invention [Problem to be solved by the invention]

[0006] The applicant of the present application has invented a method for re-liquefying evaporated gas without using any other refrigerant by using the evaporated gas itself as the refrigerant, in which the evaporated gas compressed by the compressor is cooled by heat exchange with the evaporated gas before being compressed by the compressor, and then expanded by a JT valve or the like to re-liquefy a part of the evaporated gas. This type of system is called a PRS (Partial Re-liquefaction System).

[0007] For example, when the amount of evaporated gas to be reliquefied is large, such as when the amount of liquefied gas in the storage tank is large and therefore the amount of evaporated gas generated is also large, or when the ship is at anchor or when the amount of evaporated gas used by the engine is small due to slow operation, the PRS alone may not be able to achieve the required amount of reliquefaction. Therefore, the applicant of this application has invented an improved technology for the PRS that enables a larger amount of evaporated gas to be reliquefied.

[0008] As an improved technology of PRS, a system that can additionally cool evaporative gas by using the evaporative gas itself as a refrigerant in a refrigeration cycle is called MRS (Methane Refrigeration System).

[0009] Also, other refrigerants such as mixed refrigerants or nitrogen can be used to cool the evaporated gas to be reliquefied.

[0010] When using a reliquefaction cycle to reliquefy evaporated gas on a ship, typical liquefaction methods include the SMR cycle and the C3MR cycle. The C3MR cycle (Propane-precooled Mixed Refrigerant Cycle) is a process in which natural gas is cooled using a single refrigerant, propane, and then liquefied and supercooled using a mixed refrigerant. The SMR cycle (Single Mixed Refrigerant Cycle) is a process in which natural gas is liquefied using a mixed refrigerant composed of multiple components.

[0011] These SMR and C3MR cycles include a process that uses a mixed refrigerant, and as the liquefaction process progresses, the refrigerant leaks and the composition ratio of the mixed refrigerant changes, decreasing the liquefaction efficiency. Therefore, it is necessary to continuously measure the composition ratio of the mixed refrigerant and maintain the refrigerant composition by filling in the missing refrigerant components.

[0012] Another method of reliquefaction cycle to reliquefy the evaporated gas is a single cycle liquefaction process using nitrogen refrigerant.

[0013] Compared to cycles that use mixed refrigerants, nitrogen refrigerants have a lower cooling efficiency, but they have the advantage of being safer because the refrigerant is an inert substance, and they are easy to apply to ships because there is no phase change of the refrigerant.

[0014] On the other hand, in a ship equipped with an engine that can use evaporative gas as fuel, a compressor that supplies fuel to the engine can be used to re-liquefy the evaporative gas. Such a compressor is provided according to the fuel supply conditions required by the engine, and in order to prevent damage to the equipment, the temperature of the evaporative gas supplied to the compressor must be within the range of the appropriate intake temperature required by the compressor.

[0015] In addition, when a conventional reliquefaction system is operated, the reliquefied evaporated gas is separated into gas and liquid, and the reliquefied gas is then collected in a storage tank, and the separated gas is resupplied to the reliquefaction system together with the evaporated gas generated in the storage tank.

[0016] However, the evaporated gas generated in the storage tank contains components other than methane, such as nitrogen, which has a lower liquefaction point than methane and cannot be liquefied in the reliquefaction device. Therefore, if the reliquefaction device is operated continuously, the nitrogen content in the reliquefaction cycle will gradually increase, causing a decrease in reliquefaction performance.

[0017] The present invention solves these problems and proposes a system that can effectively cool the evaporated gas being reliquefied, thereby improving reliquefaction performance, by adjusting the temperature of the evaporated gas within the range of the appropriate suction temperature required by the compressor.

[0018] The present invention also proposes a method for maintaining the reliquefaction performance of the reliquefaction device by discharging and treating the off-gas with a high nitrogen content separated by gas-liquid separation in the reliquefaction device. [Means for solving the problem]

[0019] In order to solve the above problems, the present invention provides a ship's evaporated gas re-liquefaction system, comprising a compressor that compresses evaporated gas generated from liquefied gas stored in a storage tank of the ship, a heat exchanger that cools the compressed gas compressed by the compressor, a refrigerant circulation line that circulates refrigerant supplied to the heat exchanger, a heating line that connects the storage tank and the compressor, and a heater provided in the heating line, characterized in that the heater heats the evaporated gas to an appropriate suction temperature required by the compressor.

[0020] It is also preferable to further include a gas supply line connecting the storage tank and the compressor via the heat exchanger, and a gas supply valve provided in the gas supply line for adjusting the flow rate of evaporated gas supplied to the heat exchanger and then supplied to the compressor, and the evaporated gas generated in the storage tank is supplied to the heat exchanger via the gas supply line to be heat exchanged with the compressed gas, and then supplied to the compressor.

[0021] It is also preferable to further provide a bypass valve provided in the heating line for adjusting the flow rate of the evaporated gas supplied to the heater and then to the compressor, and to supply the evaporated gas generated in the storage tank to the heat exchanger, heated, and then supplied to the compressor, so that when the reliquefaction system is not operated or the load on the reliquefaction system is small, all or a part of the evaporated gas generated in the storage tank is bypassed by the heat exchanger, heated by the heater via the heating line, and then supplied to the compressor.

[0022] The present invention further includes a refrigerant compression section that is provided in the refrigerant circulation line and compresses the refrigerant discharged from the heat exchanger after heat exchange in the heat exchanger, and a refrigerant expansion device that is provided in the refrigerant circulation line and expands and cools the refrigerant supplied to the heat exchanger, and it is preferable that the refrigerant in the refrigerant circulation line is compressed in the refrigerant compression section, supplied to the heat exchanger and cooled, and then expanded and cooled in the refrigerant expansion device and supplied to the heat exchanger as a cold source.

[0023] It is also preferable that the heat exchanger exchanges heat among four flows: compressed gas compressed by the compressor, refrigerant cooled by expansion in the refrigerant expansion device, uncompressed evaporated gas supplied to the compressor from the storage tank via the gas supply line, and refrigerant compressed in the refrigerant compression section.

[0024] In addition, it is preferable that the refrigerant compression unit is connected to the refrigerant expansion device and compresses the refrigerant by receiving expansion energy of the refrigerant from the refrigerant expansion device.

[0025] It is also preferable that the compressor compresses the evaporative gas to a fuel supply pressure required by a propulsion engine provided on the ship, and supplies the evaporative gas compressed to a pressure of 10 bara to 20 bara to the propulsion engine.

[0026] It is also preferable to further include a pressure reducing device which reduces the pressure of the compressed gas cooled by heat exchange in the heat exchanger, and a gas-liquid separator which separates the evaporated gas reduced in pressure by the pressure reducing device into gas and liquid, and to merge the flash gas separated in the gas-liquid separator with the flow of the uncompressed evaporated gas upstream of the heat exchanger, and to send the liquefied gas separated in the gas-liquid separator to the storage tank.

[0027] In addition, in order to solve the above-mentioned problems, the present invention provides an off-gas treatment system for a reliquefaction device for a ship, comprising: a compressor that compresses evaporated gas generated from liquefied gas stored in a storage tank of a ship; a reliquefaction line that connects the compressor to the storage tank and reliquefies the evaporated gas and sends it to the storage tank; a heat exchanger that is provided in the reliquefaction line and cools the evaporated gas compressed by the compressor; a separator that is provided in the reliquefaction line and separates the liquefied gas sent to the storage tank from the evaporated gas cooled by the heat exchanger into gas and liquid; an off-gas combustion line that supplies the off-gas separated by the separator to a gas combustion unit; and a vapor main line that discharges the evaporated gas generated in the storage tank from the storage tank, wherein the gas combustion unit is supplied with the evaporated gas from the vapor main line and combusts the off-gas.

[0028] It is also preferable to further include a heater provided in the off-gas combustion line for heating the off-gas supplied to the gas combustion unit, an off-gas recirculation line branching off from the off-gas combustion line upstream of the heater of the off-gas combustion line and connecting to the vapor main line, and an overpressure prevention valve provided in the off-gas recirculation line.

[0029] It is also preferable that when the gas combustion unit is started up or shut down, the overpressure prevention valve is opened and the off-gas is supplied to the vapor main line via the off-gas recirculation line.

[0030] The heat exchanger preferably further comprises a refrigerant circulation section through which a refrigerant that exchanges heat with the evaporated gas in the heat exchanger circulates, the refrigerant in the refrigerant circulation section being nitrogen.

[0031] It is also preferable that the refrigerant circulation system further includes a first valve provided upstream of a branch point of the off-gas combustion line with the off-gas recirculation line, a pressure compensation line branching off from the reliquefaction line downstream of the compressor and connected to an upper portion of the separator, and a backup line supplying nitrogen to the pressure compensation line from a buffer tank provided in the refrigerant circulation section, and that the internal pressure of the separator is adjusted by supplying evaporated gas or nitrogen to the separator via the pressure compensation line, or discharging the off-gas from the separator via the first valve.

[0032] It is also preferable to further include a gas supply line connecting the vapor main line and an inboard engine, send the off-gas to the vapor main line via the off-gas recirculation line, and supply the off-gas to the inboard engine as fuel together with evaporated gas discharged from the storage tank.

[0033] It is also preferable to further include a liquefied gas supply line connecting the storage tank and the gas supply line, and a vaporizer provided in the liquefied gas supply line for vaporizing the liquefied gas supplied from the storage tank, and when a mixed gas of the off-gas and the evaporated gas discharged from the storage tank does not satisfy the heat generation amount of the engine, the liquefied gas stored in the storage tank is forcibly vaporized and mixed with the mixed gas.

[0034] In order to solve the above problems, the present invention provides a method for re-liquefying evaporated gas in a ship, characterized in that evaporated gas generated in a storage tank of the ship is compressed by a compressor, then cooled by heat exchange in a heat exchanger to which a refrigerant circulated via a refrigerant circulation line is supplied, thereby re-liquefying the evaporated gas, and the evaporated gas generated in the storage tank is heated by a heater to an appropriate suction temperature required by the compressor, and then supplied to the compressor.

[0035] In addition, the evaporated gas generated in the storage tank is heated by heat exchange in the heat exchanger and then supplied to the compressor. When a reliquefaction system that reliquefies the evaporated gas is not operated or the load on the reliquefaction system is small, all or a part of the evaporated gas generated in the storage tank is bypassed by the heat exchanger and heated by a heater provided in a heating line connecting the storage tank and the compressor, and then supplied to the compressor.

[0036] In addition, the refrigerant circulating through the refrigerant circulation line is compressed in a refrigerant compression section, cooled in the heat exchanger, and then expanded and cooled in a refrigerant expansion device and supplied to the heat exchanger as a cold source. The refrigerant compression section is connected to the refrigerant expansion device, and the expansion energy of the refrigerant is transmitted from the refrigerant expansion device to compress the refrigerant.

[0037] The compressor compresses the evaporated gas to a fuel supply pressure required by a propulsion engine installed in the ship, and supplies the evaporated gas compressed to a pressure of 10 bara to 20 bara to the propulsion engine.

[0038] In addition, in order to solve the above-mentioned problems, the present invention provides an off-gas treatment method for a re-liquefaction device for a ship, which is characterized in that evaporated gas generated from liquefied gas stored in a storage tank of a ship is compressed by a compressor, the evaporated gas compressed by the compressor is cooled and re-liquefied by a heat exchanger, and after gas-liquid separation by a separator, the separated liquefied gas is sent to the storage tank, and the off-gas separated by the separator is supplied to a gas combustion unit together with evaporated gas generated in the storage tank and discharged to a vapor main line, and the off-gas is combusted.

[0039] It is also preferable that, when the gas combustion unit is started up or shut down, the off-gas separated by the separator is supplied to the vapor main line.

[0040] It is also preferable to mix the off-gas supplied to the vapor main line with evaporated gas discharged from the storage tank to the vapor main line or with vaporized gas obtained by forcibly vaporizing the liquefied gas stored in the storage tank, and supply the mixture as fuel to the inboard engine in accordance with the amount of heat generated by the inboard engine. Effect of the Invention

[0041] In the present invention, the extremely low temperature uncompressed evaporated gas generated in the storage tank is supplied to a heat exchanger and then to a compressor, where it is used as a cold source for the heat exchanger together with the refrigerant in the refrigerant circulation line, and the extremely low temperature evaporated gas can be supplied to the compressor according to the appropriate suction temperature required by the compressor. Furthermore, by providing a heating line that allows the evaporated gas generated in the storage tank to bypass the heat exchanger and be directly supplied to the compressor, and by providing a heater in the heating line, the evaporated gas can be heated to the appropriate suction temperature and supplied to the compressor even when the reliquefaction system is not in operation or the load on the reliquefaction system is small.

[0042] In this way, the cooling efficiency of the heat exchanger can be improved by utilizing the cold energy of the evaporation gas itself and the cold energy of the refrigerant cycle. As a result, it is not necessary to install and operate additional equipment such as a boost compressor that compresses the evaporation gas to be reliquefied to a high pressure in order to increase the reliquefaction rate, and CAPEX (Capital Expenditure) and OPEX (Operating Expenditure) can be reduced. In addition, regardless of the operating status and load of the reliquefaction system, the evaporation gas is supplied to the compressor at the appropriate suction temperature required by the compressor, thereby preventing damage to the compressor equipment and enabling stable operation.

[0043] In addition, by re-liquefying only the evaporative gas remaining after fuel consumption, the load on the refrigerant cycle can be adjusted according to the amount of remaining evaporative gas, thereby reducing fuel consumption.

[0044] In the present invention, the evaporated gas generated from the liquefied gas stored in the storage tank is reliquefied, thereby preventing the loss of LNG and increasing the transportation efficiency of LNG.

[0045] In particular, by discharging the off-gas, which has become highly nitrogen-content due to continuous operation of the reliquefaction apparatus, from the reliquefaction apparatus and treating it, the reliquefaction apparatus can be operated stably while maintaining its reliquefaction performance.

[0046] Furthermore, off-gas has a high nitrogen content, making it difficult to incinerate it or use it as fuel, and it also contains methane gas, making it unacceptable to vent it to the atmosphere.By solving these problems, the system can flexibly and effectively treat off-gas according to the ship's conditions. [Brief description of the drawings]

[0047] [Figure 1] 1 is a schematic diagram of a marine evaporative gas reliquefaction system according to an embodiment of the present invention. [Diagram 2] 1 illustrates a schematic diagram of an off-gas treatment system for a marine reliquefaction unit according to an embodiment of the present invention. [Diagram 3] FIG. 3 shows a first embodiment of the off-gas treatment system. [Figure 4] A second embodiment of the off-gas treatment system shown in FIG. 2 is shown. [Diagram 5] 3 shows a third embodiment of the off-gas treatment system shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] The operating advantages of the present invention and the objects attained by the embodiments of the present invention will now be described by way of example with reference to the drawings and the contents thereof.

[0049] Hereinafter, the configuration and operation of the embodiment of the present invention will be described with reference to the drawings. In addition, the reference numerals of the components in each drawing are denoted by the same numerals as far as possible when the same components are shown in other drawings.

[0050] Hereinafter, the term "ship" as used herein includes ships equipped with engines that use liquefied gas or evaporated gas generated from liquefied gas as fuel for propulsion or power generation engines, and all types of ships that use liquefied gas or evaporated gas as fuel for their onboard engines. Representative examples include ships with self-propelling capabilities, such as LNG carriers, liquid hydrogen carriers, and LNG RVs (regasification vessels), as well as floating marine structures that do not have propulsion capabilities, such as LNG FPSOs (floating production storage offloading) and LNG FSRUs (floating storage reasification units).

[0051] The liquefied gas of the present invention includes all kinds of liquefied gases that can be liquefied at low temperatures for transportation, generate evaporated gases in a stored state, and can be used as fuel for engines, etc. Examples of such liquefied gases include liquefied petrochemical gases such as LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), liquefied ethylene gas, and liquefied propylene gas. However, in the embodiment described below, LNG, which is one of the representative liquefied gases, will be described as an example.

[0052] FIG. 1 is a schematic diagram showing a marine evaporative gas reliquefaction system according to a first embodiment of the present invention.

[0053] As shown in FIG. 1, the reliquefaction system of this embodiment includes compressors 100a and 100b. The compressors 100a and 100b are for reliquefying evaporated gas generated from liquefied gas stored in a storage tank T provided on the ship, and the evaporated gas generated in the storage tank T is supplied to the compressors 100a and 100b and compressed. The reliquefaction system also includes a heat exchanger 200, and all or a part of the evaporated gas compressed by the compressors 100a and 100b is supplied to the heat exchanger 200 and cooled by heat exchange using the uncompressed evaporated gas before being supplied to the compressors 100a and 100b as a refrigerant. For this reason, a gas supply line GL that connects the storage tank T and the compressors 100a and 100b via the heat exchanger 200, and a reliquefaction line RL that is provided downstream of the compressors 100a and 100b and supplies the reliquefied evaporated gas to the storage tank T are provided.

[0054] In addition, a refrigerant circulation line CL is provided through which the refrigerant supplied to the heat exchanger 200 circulates. The refrigerant circulation line CL is provided with a refrigerant expansion device 650 that expands and cools the refrigerant supplied to the heat exchanger 200, and a refrigerant compression section 600 that compresses the refrigerant discharged from the heat exchanger 200 after heat exchange in the heat exchanger 200.

[0055] The refrigerant compression unit 600 is provided with a compander type compressor, and the compressor and the refrigerant expansion device 650 are axially connected to each other, and the expansion energy of the refrigerant is transmitted to drive the compander type compressor. In this embodiment, the refrigerant compression unit 600 is driven by a motor M, but the refrigerant expansion device 650 may be connected to a motor M, and the expansion energy of the refrigerant may be transmitted to drive the motor M to compress the refrigerant.

[0056] The refrigerant compressed in the refrigerant compression section 600 is supplied to the heat exchanger 200 and cooled, and then supplied to the refrigerant expansion device 650 via the refrigerant circulation line CL, where it is cooled by expansion, and is supplied again to the heat exchanger 200 as a refrigerant.

[0057] Therefore, in the heat exchanger 200 of this embodiment, heat is exchanged between four flows: all or a portion of the evaporative gas compressed in the compressors 100a, 100b, the uncompressed evaporative gas before being supplied to the compressors 100a, 100b, the refrigerant cooled by expansion in the refrigerant expansion device 650, and the refrigerant compressed in the refrigerant compression section 600.

[0058] The refrigerant circulated through the refrigerant circulation line CL and supplied to the heat exchanger 200 is, for example, nitrogen (N2). In a refrigerant cycle in which a compressed refrigerant is supplied to the heat exchanger 200, cooled by the cold energy of the refrigerant itself, expanded, and circulated, and the evaporated gas is cooled by heat exchange, a large amount of nitrogen refrigerant is required to cool the evaporated gas to a liquefaction temperature because there is a difference in heat capacity between the evaporated gas, whose main component is methane, and nitrogen. For this reason, most of the cold energy of the refrigerant cycle must be used to cool the nitrogen refrigerant itself, which increases the capacity of the refrigerant compression device and expansion device, and increases the power consumption associated with these increases in capacity. In order to solve this problem, in this embodiment, the extremely low temperature uncompressed evaporated gas generated in the storage tank T is supplied to the heat exchanger 200 and then supplied to the compressors 100a and 100b. This reduces the refrigerant flow rate required for the refrigerant cycle, which in turn reduces the capacity of the device required for refrigerant compression and expansion, reduces power consumption, and further reduces installation and operation costs.

[0059] In the system of this embodiment, in the process of processing the evaporated gas generated from the liquefied gas in the storage tank T, the evaporated gas generated in the storage tank T is supplied to the heat exchanger 200 and then supplied to the compressors 100a, 100b.

[0060] In the compressors 100a and 100b, the evaporated gas is compressed, for example, to a fuel supply pressure for the ship's main engine or propulsion engine. For example, the evaporated gas is compressed to a pressure of 5.5 barg when a DF engine is provided, 15 barg when an X-DF engine is provided, and 300 barg when an ME-GI engine is provided. The compressed evaporated gas is supplied as fuel to the ship's propulsion engine E1, power generation engine E2, etc., and the remaining evaporated gas that is not supplied as fuel is reliquefied.

[0061] Regarding regulations for ships, the compressor that supplies fuel to the engine must be designed for redundancy in case of an emergency. Redundancy means that when one device cannot be used due to a breakdown or maintenance, the other device can be used instead. For this purpose, the compressor is configured with a main compressor 100a and a standby compressor 100b, and in normal operation, the main compressor 100a, i.e., one compressor, is operated to supply fuel to the propulsion engine E1 and the power generation engine E2, and the remaining amount of compressed gas compressed by the main compressor 100a is reliquefied in the reliquefaction line RL.

[0062] The evaporated gas compressed by the compressors 100a and 100b is supplied to the heat exchanger 200 via a re-liquefaction line RL and cooled. The evaporated gas re-liquefied after compression and the refrigerant compressed by the refrigerant compression unit 600 become a high-temperature fluid (hot stream) of the heat exchanger 200, while the uncompressed evaporated gas and the refrigerant cooled by expansion in the refrigerant expansion device 650 become a low-temperature fluid (cold stream) of the heat exchanger 200.

[0063] In the heat exchanger 200, the four flows are heat exchanged, and the high-temperature fluid is cooled by heat exchange with the low-temperature fluid. The heat exchanger 200 is, for example, a BAHE (Brazed Aluminum Heat Exchanger).

[0064] The supply and discharge locations of each stream within the heat exchanger 200 may be different to more effectively exchange heat between the hot and cold fluids and cool the compressed gas being reliquefied.

[0065] Of the low-temperature fluids in the heat exchanger 200, the nitrogen refrigerant that is cooled by expansion and then supplied to the heat exchanger 200 has a temperature of about -167°C when the pressure is about 10 bar, which is lower than the temperature of the other low-temperature fluid in the heat exchanger 200, the uncompressed evaporated gas (about -50°C). Therefore, if both are supplied to the heat exchanger 200, not all of the cold energy of the nitrogen refrigerant is used to cool the compressed gas to be reliquefied, and there is a risk that a part of the cold energy is absorbed by the other low-temperature fluid (the uncompressed evaporated gas). For this reason, although not particularly illustrated or described, the flow CL of the nitrogen refrigerant, which has a low temperature among the low-temperature fluids, is supplied from the downstream of the heat exchanger 200 and passes through the entire length of the heat exchanger 200, and the flow GL of the uncompressed evaporated gas, which has a high temperature among the low-temperature fluids, is supplied from the middle part of the heat exchanger 200.

[0066] Therefore, the compressed gas in the reliquefaction line RL is cooled successively as it passes from the high temperature region to the low temperature region of the heat exchanger 200, and in the high temperature region, it is cooled by cold energy supplied from two low temperature fluids, i.e., the refrigerant in the refrigerant circulation line CL and the uncompressed evaporated gas in the evaporated gas supply line. Also, in the low temperature region, it is cooled successively by heat exchange with one low temperature fluid, the refrigerant in the refrigerant circulation line CL immediately after being supplied to the heat exchanger 200.

[0067] By exchanging heat in this manner, the compressed gas being reliquefied can be cooled more effectively, increasing the reliquefaction rate, and preventing thermal fatigue of the heat exchanger 200, thereby preventing damage to the equipment.

[0068] Meanwhile, the evaporated gas (compressed gas) cooled by heat exchange in the heat exchanger 200 is supplied to the pressure reducing device 400 in the reliquefaction line RL and reduced in pressure, and the evaporated gas reduced in pressure reducing device 400 is supplied to the gas-liquid separator 500.

[0069] The pressure reducing device 400 is composed of an expander or an expansion valve such as a Joule-Thomson valve that reduces the pressure of the evaporated gas cooled after compression. The evaporated gas is cooled by adiabatic expansion or isentropic expansion due to the reduction in pressure.

[0070] The evaporated gas that has been depressurized and additionally cooled by the depressurizing device 400 is supplied to the gas-liquid separator 500. The liquid separated by the gas-liquid separator 500 is supplied to the storage tank T via the re-liquefaction line RL and stored again. However, in this embodiment, even if the gas-liquid separator 500 is used, complete phase separation between the flash gas in gas form and the liquefied gas in liquid form may not occur, and the separated liquid or liquefied gas may contain unseparated flash gas.

[0071] The flash gas separated in the gas-liquid separator 500 is joined to the flow of uncompressed evaporative gas upstream of the heat exchanger 200 and the heater 300 from the top of the gas-liquid separator 500 via the flash gas line FL, and is supplied to the heat exchanger 200 or the heater 300, and then supplied to the compressors 100a, 100b.

[0072] The system of this embodiment utilizes the cold energy of the evaporative gas itself and the cold energy of the refrigerant cycle to increase the cooling efficiency of the heat exchanger 200, thereby eliminating the need to install and operate additional equipment such as a boost compressor that compresses the re-liquefied evaporative gas to high pressure in order to increase the re-liquefaction rate, thereby reducing CAPEX (capital expenditure) and OPEX (operating expenditure).

[0073] On the other hand, the evaporated gas generated in the storage tank T is discharged from the storage tank T at an extremely low temperature in the range of -140°C to -100°C depending on the storage tank operation. At this time, depending on the type of compressor provided for supplying fuel to the engine, the temperature of the evaporated gas supplied to the compressor is required to be within a predetermined temperature range. For example, a normal temperature compressor is installed as a compressor for supplying fuel to a medium pressure engine such as an X-DF engine. In this case, when the reliquefaction system is operating and the amount of evaporated gas to be reliquefied is large and the load of the reliquefaction system is equal to or greater than a predetermined range, the low temperature evaporated gas generated in the storage tank T is supplied to the heat exchanger 200, where it is sufficiently heated by heat exchange and supplied to the compressor. However, when the amount of evaporated gas consumed by the engine is large and the reliquefaction system is not operated or the load of the reliquefaction system is small, even if the evaporated gas is supplied to the heat exchanger 200, the evaporated gas is not sufficiently heated to the appropriate intake temperature required by the compressor.

[0074] In order to solve such problems, the system of this embodiment provides a heating line BL that bypasses the heat exchanger 200 from the storage tank T and supplies gas directly to the compressors 100a, 100b, and a heater 300 that can heat the evaporated gas is provided in this heating line BL.

[0075] The gas supply line GL is provided with a gas supply valve GV that adjusts the flow rate of the evaporated gas that is supplied to the compressors 100a and 100b after being supplied to the heat exchanger 200. In addition, the heating line BL is provided with a bypass valve BV that adjusts the flow rate of the evaporated gas that is supplied to the heater 300 and then to the compressors 100a and 100b.

[0076] When the reliquefaction system is operated, the evaporated gas generated in the storage tank T is heated by heat exchange in the heat exchanger 200 and then supplied to the compressors 100a, 100b. However, when the reliquefaction system is not operated or the load of the reliquefaction system is small, all or a part of the evaporated gas generated in the storage tank T bypasses the heat exchanger and passes through the heating line BL to be heated in the heater 300 and supplied to the compressors 100a, 100b.

[0077] By adjusting the opening of the gas supply valve GV and the bypass valve BV and adjusting the flow rate of the evaporated gas supplied to the compressors 100a, 100b after being supplied to the heat exchanger 200 and the heater 300, respectively, the evaporated gas can be supplied to the compressors 100a, 100b at the appropriate suction temperature required by the compressors 100a, 100b even when the reliquefaction system is not operated or the load of the reliquefaction system is small. In this way, by supplying the evaporated gas to the compressors 100a, 100b at the appropriate suction temperature required by the compressors 100a, 100b regardless of the operating status and load of the reliquefaction system, damage to the compressors 100a, 100b can be prevented and stable operation can be achieved.

[0078] Fig. 2 is a schematic diagram showing an off-gas treatment system for a marine reliquefaction unit according to a second embodiment of the present invention, Fig. 3 to Fig. 5 each show various examples of operation of the system according to this embodiment.

[0079] As shown in Figure 2, the off-gas treatment system of the reliquefaction equipment in this embodiment is an off-gas treatment system of the reliquefaction equipment for reliquefying evaporated gas generated from liquefied gas stored in a storage tank CT provided on board the ship and returning it to the storage tank CT, and is equipped with a compressor 150 which is supplied with evaporated gas and compresses the evaporated gas, and a reliquefaction line RL which connects the compressor 150 to the storage tank CT and reliquefies the evaporated gas and returns it to the storage tank CT.

[0080] The evaporative gas generated in the storage tank CT is discharged into the vapor main line VM, and is supplied as fuel to the onboard engine E from the vapor main line VM via the gas supply line GL.

[0081] A FG compressor (Fuel Gas Compressor) 100 that compresses evaporated gas to the fuel supply pressure of the inboard engine E is provided in the gas supply line GL.

[0082] In the FG compressor 100, the evaporated gas is compressed to a pressure of, for example, 5.5 barg when a DF engine is installed, 15 barg when an X-DF engine is installed, and 300 barg when an ME-GI engine is installed. The compressed evaporated gas is supplied as fuel to the inboard engines E, and the remaining evaporated gas that is not supplied as fuel is reliquefied via a reliquefaction line RL.

[0083] The compressor 150 of the reliquefaction line RL can additionally compress the evaporated gas compressed by the FG compressor 100 in order to increase the reliquefaction rate of the evaporated gas. If there is no need to additionally compress the evaporated gas compressed by the FG compressor 100 for reliquefaction, the compressor 150 does not need to be installed.

[0084] The evaporated gas compressed by the compressor 150 is supplied to the heat exchanger 200 via a re-liquefaction line RL, and is cooled by heat exchange with a refrigerant.

[0085] The reliquefaction line RL is provided with a heat exchanger 200 that cools the evaporated gas compressed by the compressor 150, and a separator 300 that separates the evaporated gas cooled by the heat exchanger 200 into gas and liquid, and the liquefied gas separated by the separator 300 is supplied to a storage tank. If necessary, the evaporated gas cooled by the heat exchanger 200 may be supplied to a pressure reducing device (not shown) to reduce the pressure, and then supplied to the separator 300.

[0086] In the heat exchanger 200, the refrigerant circulating in a refrigerant circulation section (not shown) and the uncompressed evaporative gas generated in the storage tank CT are used as cold heat sources, and the evaporative gas is cooled by heat exchange.

[0087] The refrigerant circulation unit includes a refrigerant circulation line through which the refrigerant circulates, and nitrogen (N2) can be used as the refrigerant circulating through the refrigerant circulation line. After undergoing compression, cooling, and cooling by expansion through the refrigerant circulation line, the nitrogen is used as the refrigerant for the heat exchanger 200, and is compressed again to circulate through the refrigerant circulation line.

[0088] The evaporated gas cooled in the heat exchanger 200 is supplied to the separator 300 via the re-liquefaction line RL, and the separated re-liquefied gas is sent from the separator 300 to the storage tank CT by opening and closing the liquid level control valve installed downstream of the separator 300.

[0089] When the liquid level control valve downstream of the separator 300 is opened to send the liquefied gas from the separator 300 to the storage tank, the internal pressure of the separator 300 changes. In this case, the internal pressure of the separator 300 can be maintained by the flash gas, i.e., off-gas, generated from the liquefied gas supplied to the separator 300.

[0090] In this case, if the liquefied gas cooled by heat exchange with the nitrogen refrigerant in the refrigerant circulation section is supercooled and supplied to the separator 300, the internal pressure of the separator 300 may drop suddenly if the liquid level control valve downstream of the separator 300 is opened in a state where no off-gas is generated or the amount of generated off-gas is small. In this embodiment, in order to compensate for the pressure of the separator 300 and maintain the internal pressure even in such a case, a pressure compensation line PL is provided downstream of the compressor 150, branched from the re-liquefaction line RL, and connected to the upper part of the separator 300, and a backup line BL is provided to supply nitrogen to the pressure compensation line PL. As a result, when the liquefied gas is sent from the separator 300 to the storage tank CT, the internal pressure of the separator 300 can be maintained by supplying evaporated gas or nitrogen to the separator 300 through the pressure compensation line PL.

[0091] A pressure detector PI that detects the pressure inside the separator 300 and a liquid level detector LI that detects the liquid level inside the separator 300 are provided, and a liquid level controller LIC that opens and closes the liquid level adjustment valve according to the liquid level value detected by the liquid level detector LI is provided. A pressure compensation valve PV is provided downstream of the junction of the pressure compensation line PL with the backup line BL. A first shutoff valve SV1 is provided upstream of the junction of the pressure compensation line PL with the backup line BL. A second shutoff valve SV2 is provided in the backup line BL.

[0092] In response to the internal pressure of the separator 300 detected by the pressure detector PI, the pressure controller PIC adjusts the pressure of the evaporated gas or nitrogen through the pressure compensation valve PV, and supplies the evaporated gas or nitrogen to the top of the separator 300 through the pressure compensation line PL.

[0093] The nitrogen supplied to the separator 300 via the backup line BL can be supplied from a nitrogen buffer tank (N2Buffer Tank) of a nitrogen supply system (N2Supply System) installed on the ship, or from a nitrogen inventory system (N2Inventory System) that supplies and replenishes the nitrogen refrigerant circulating in the refrigerant circulation section.

[0094] However, if the re-liquefaction device is operated continuously, nitrogen, which has a lower liquefaction point than methane, is not liquefied in the re-liquefaction device and vaporizes first when the temperature changes. In addition, nitrogen may be supplied for pressure adjustment of the separator 300, and the nitrogen content in the evaporated gas discharged from the storage tank CT gradually increases, causing a decrease in re-liquefaction performance. Even if off-gas with a high nitrogen content is separated from the separator 300, the high nitrogen content does not satisfy the heat generation amount of the onboard engine E, making it difficult to supply it as fuel. Moreover, since off-gas with a high nitrogen content contains methane, it is not permitted to vent it directly into the atmosphere.

[0095] In order to solve these problems and effectively treat the off-gas, the system of this embodiment is provided with an off-gas combustion line OSL that supplies the off-gas separated in the separator 300 to a gas combustion unit GCU (Gas Combustion Unit).

[0096] As a result, evaporated gas is supplied to the gas combustion unit GCU from the vapor main line VM and burned together with the off-gas.

[0097] Further, the off-gas combustion line OSL is provided with a heater 400 for heating the off-gas supplied to the gas combustion unit GCU. Further, an off-gas recirculation line FL is provided upstream of the heater 400, branching from the off-gas combustion line OSL and connecting to the vapor main line VM, and an overpressure prevention valve OV3 is provided in the off-gas recirculation line FL.

[0098] A first valve OV1 is provided upstream of the branch point of the offgas combustion line OSL with the offgas recirculation line FL, and can adjust the discharge of offgas from the separator 300 to the offgas combustion line OSL, the offgas recirculation line FL, etc.

[0099] A liquefied gas supply line LL is provided to connect the storage tank CT and the gas supply line GL, and a vaporizer 500 that vaporizes the liquefied gas supplied from the storage tank CT is provided in the liquefied gas supply line LL.

[0100] An example of off-gas treatment in the system of this embodiment will be described with reference to Figures 3 to 5. First, the off-gas separated by the separator 300 as in the first example shown in Figure 3 is supplied to the gas combustion unit GCU via the off-gas combustion line OSL and the heater 400. Then, depending on the amount of off-gas supplied to the GCU, evaporative gas (NBOG) capable of burning the off-gas is supplied to the gas combustion unit GCU via the vapor main line VM, and the off-gas is incinerated.

[0101] When the gas combustion unit GCU is stopped from operating due to startup (Start) or trip (Trip), the overpressure prevention valve OV3 is opened and the off-gas is supplied to the vapor main line VM via the off-gas recirculation line FL to operate the reliquefaction device.

[0102] As another example, in a second example shown in Fig. 4, the off-gas separated by the separator 300 can be supplied and treated as fuel for the inboard engine E. In this case, as described above, the off-gas may not meet the heat generation amount of the inboard engine E due to its high nitrogen content. In this embodiment, to solve this problem, the off-gas is sent to the gas supply line GL via the off-gas recirculation line FL, and evaporated gas (NBOG) discharged from the storage tank CT to the vapor main line VM is mixed with the off-gas, which is then supplied to the FG compressor 100 according to the heat generation amount of the inboard engine E, compressed, and then supplied to the inboard engine E.

[0103] On the other hand, since the amount of off-gas is greater than the amount of evaporated gas generated in the storage tank CT, there are cases where the naturally generated evaporated gas alone is not enough to generate the heat of the inboard engine E. Therefore, in the third embodiment shown in Fig. 5, in this case, the liquefied gas in the storage tank CT is supplied to the vaporizer 500 via the liquefied gas supply line LL for forced vaporization, the vaporized liquefied gas is sent to the gas supply line GL, the vaporized liquefied gas is mixed with the off-gas or a mixed gas of the off-gas and evaporated gas (NBOG), the mixed gas is supplied to the FG compressor 100 for compression, and then supplied to the inboard engine E as fuel.

[0104] As described above, in this embodiment, the evaporated gas generated in the storage tank CT can be reliquefied to improve transportation efficiency, and the off-gas with a high nitrogen content due to continuous operation of the reliquefaction device can be discharged and effectively treated, allowing stable operation while maintaining the reliquefaction performance of the reliquefaction device.

[0105] The present invention is not limited to the above-described embodiment, and it will be apparent to those skilled in the art to which the present invention pertains that various changes or modifications can be made without departing from the technical gist of the present invention.

Claims

1. A compressor for compressing evaporated gas generated from liquefied gas stored in a storage tank of the ship; and A heat exchanger for cooling the compressed gas compressed by the compressor; and a refrigerant circulation line for circulating a refrigerant to be supplied to the heat exchanger; and a heating line connecting the storage tank and the compressor; and A heater provided in the heating line; and a gas supply line connecting the storage tank and the compressor via the heat exchanger; and a gas supply valve provided in the gas supply line for adjusting a flow rate of the evaporated gas supplied to the heat exchanger and then to the compressor; and a bypass valve provided in the heating line for adjusting a flow rate of the evaporated gas supplied to the heater and then to the compressor; The evaporated gas generated in the storage tank is supplied to the heat exchanger via the gas supply line to exchange heat with the compressed gas, and then supplied to the compressor; The heater heats the evaporative gas to a proper suction temperature required by the compressor, A ship's evaporated gas reliquefaction system, characterized in that when the reliquefaction system is not operated or the load on the reliquefaction system is small, all or part of the evaporated gas generated in the storage tank is bypassed by the heat exchanger, heated by the heater via the heating line, and then supplied to the compressor.

2. A compressor for compressing evaporated gas generated from liquefied gas stored in a storage tank of a ship; and a reliquefaction line connecting the compressor and the storage tank, reliquefying the evaporated gas and sending the reliquefied evaporated gas to the storage tank; and a heat exchanger provided in the reliquefaction line for cooling the evaporated gas compressed by the compressor; and a separator provided in the reliquefaction line for separating the liquefied gas sent to the storage tank from the evaporated gas cooled by the heat exchanger into gas and liquid; and an off-gas combustion line that supplies the off-gas separated by the separator to a gas combustion unit; and A vapor main line for discharging evaporated gas generated in the storage tank from the storage tank; and an off-gas recirculation line branching off from the off-gas combustion line and connecting to the vapor main line; and a refrigerant circulation section in which a refrigerant that exchanges heat with the evaporated gas in the heat exchanger circulates; and a first valve provided upstream of a branch point of the off-gas combustion line with the off-gas recirculation line; and a pressure compensation line branching off from the reliquefaction line downstream of the compressor and connected to an upper portion of the separator; and a backup line for supplying nitrogen to the pressure compensation line from a buffer tank provided in the refrigerant circulation section; The internal pressure of the separator is adjusted by supplying evaporated gas or nitrogen to the separator through the pressure compensation line, or discharging the off-gas from the separator through the first valve. Off-gas treatment system for marine reliquefaction plants.

3. The refrigerant in the refrigerant circulation section is nitrogen. The off-gas treatment system of claim 2 .

4. a refrigerant compression section provided in the refrigerant circulation line and configured to compress the refrigerant discharged from the heat exchanger after heat exchange in the heat exchanger; and a refrigerant expansion device provided in the refrigerant circulation line to expand and cool the refrigerant supplied to the heat exchanger; The refrigerant in the refrigerant circulation line is compressed in the refrigerant compression unit, supplied to the heat exchanger and cooled, and then expanded and cooled in the refrigerant expansion device, and supplied to the heat exchanger as a cold source.

2. A ship vapor reliquefaction system as claimed in claim 1.

5. The heat exchanger exchanges heat among four flows: compressed gas compressed by the compressor, refrigerant cooled by expansion in the refrigerant expansion device, non-compressed evaporated gas supplied from the storage tank to the compressor via the gas supply line, and refrigerant compressed in the refrigerant compression unit.

5. A ship vapor reliquefaction system according to claim 4.

6. The refrigerant compression unit is connected to the refrigerant expansion device, and compresses the refrigerant by receiving expansion energy of the refrigerant from the refrigerant expansion device.

5. A ship vapor reliquefaction system according to claim 4.

7. The compressor compresses the evaporated gas to a fuel supply pressure required by a propulsion engine installed in the ship, The propulsion engine is supplied with evaporated gas compressed to a pressure of 10 bara to 20 bara. A ship vapor reliquefaction system according to claim 6.

8. a pressure reducing device that reduces the pressure of the compressed gas cooled by heat exchange in the heat exchanger; and A gas-liquid separator that separates the evaporated gas decompressed by the decompression device into gas and liquid, The flash gas separated in the gas-liquid separator is merged with a flow of uncompressed evaporation gas before being supplied to the compressor upstream of the heat exchanger, and the liquefied gas separated in the gas-liquid separator is sent to the storage tank. A ship vapor reliquefaction system according to any one of claims 1 and 4 to 7.

9. A compressor for compressing evaporated gas generated from liquefied gas stored in a storage tank of the ship; and a reliquefaction line connecting the compressor and the storage tank, reliquefying the evaporated gas and sending the reliquefied evaporated gas to the storage tank; and a heat exchanger provided in the reliquefaction line for cooling the evaporated gas compressed by the compressor; and a separator provided in the reliquefaction line for separating the liquefied gas sent to the storage tank from the evaporated gas cooled by the heat exchanger into gas and liquid; and an off-gas combustion line that supplies the off-gas separated by the separator to a gas combustion unit; and A vapor main line that discharges evaporated gas generated in the storage tank from the storage tank; The gas combustion unit is supplied with evaporated gas from the vapor main line, and the off-gas is combusted together with the evaporated gas in the gas combustion unit. Off-gas treatment system for marine reliquefaction plants.

10. a heater provided in the off-gas combustion line for heating the off-gas supplied to the gas combustion unit; and an off-gas recirculation line branching off from the off-gas combustion line upstream of the heater of the off-gas combustion line and connecting to the vapor main line; and and an overpressure prevention valve provided in the off-gas recirculation line.

10. The off-gas treatment system of a marine reliquefaction unit according to claim 9.

11. When the gas combustion unit is started or stopped, the overpressure prevention valve is opened and the off-gas is supplied to the vapor main line through the off-gas recirculation line. The off-gas treatment system of a marine reliquefaction unit according to claim 10.

12. a refrigerant circulation section in which a refrigerant that exchanges heat with the evaporative gas in the heat exchanger circulates; The refrigerant in the refrigerant circulation section is nitrogen. The off-gas treatment system of a marine reliquefaction unit according to claim 10.

13. A gas supply line connecting the vapor main line and an inboard engine; The off-gas is sent to the vapor main line through the off-gas recirculation line, and the off-gas is supplied as fuel to the inboard engine together with evaporated gas discharged from the storage tank.

11. An off-gas treatment system for a marine reliquefaction plant according to claim 2 or 10.

14. a liquefied gas supply line connecting the storage tank and a gas supply line; and a vaporizer provided in the liquefied gas supply line for vaporizing the liquefied gas supplied from the storage tank; When a mixed gas of the off-gas and the evaporated gas discharged from the storage tank does not satisfy the heat generation amount of the inboard engine, the liquefied gas stored in the storage tank is forcibly vaporized and mixed with the mixed gas.

14. The marine reliquefaction unit off-gas treatment system of claim 13.

15. The evaporated gas generated in the storage tank of the ship is compressed by a compressor, and then cooled and re-liquefied by heat exchange in a heat exchanger to which a refrigerant circulating through a refrigerant circulation line is supplied. The evaporated gas generated in the storage tank is heated by heat exchange in the heat exchanger and then supplied to the compressor; When a re-liquefaction system for re-liquefying the evaporated gas is not operated or when the load of the re-liquefaction system is small, all or a part of the evaporated gas generated in the storage tank is bypassed by the heat exchanger, heated by a heater provided in a heating line connecting the storage tank and the compressor, and then supplied to the compressor. A method for reliquefying evaporated gas on a ship.

16. The refrigerant circulating through the refrigerant circulation line is compressed in a refrigerant compression section, cooled in the heat exchanger, expanded and cooled in a refrigerant expansion device, and supplied to the heat exchanger as a cold source; The refrigerant compression unit is connected to the refrigerant expansion device, and compresses the refrigerant by receiving expansion energy of the refrigerant from the refrigerant expansion device. A method for reliquefying evaporated gas in a ship according to claim 15.

17. The compressor compresses the evaporated gas to a fuel supply pressure required by a propulsion engine installed in the ship; The propulsion engine is supplied with evaporated gas compressed to a pressure of 10 bara to 20 bara. A method for reliquefying evaporated gas in a ship according to claim 16.

18. The evaporated gas generated from the liquefied gas stored in the ship's storage tank is compressed by a compressor. The evaporated gas compressed by the compressor is cooled and re-liquefied in a heat exchanger, and then separated into gas and liquid in a separator. The separated liquefied gas is then sent to the storage tank. The off-gas separated by the separator is supplied to a gas combustion unit together with the evaporated gas generated in the storage tank and discharged to a vapor main line, and the off-gas is combusted together with the evaporated gas. A method for treating off-gas from a marine reliquefaction unit.

19. The off-gas separated by the separator is supplied to the vapor main line when the gas combustion unit is started or stopped. The method for treating off-gas from a marine reliquefaction system according to claim 18.

20. the off-gas supplied to the vapor main line is mixed with evaporated gas discharged from the storage tank to the vapor main line or with vaporized gas obtained by forcibly vaporizing a liquefied gas stored in the storage tank, and the mixture is supplied as fuel to the inboard engine in accordance with a heat generation amount required by the inboard engine. The method for treating off-gas from a marine reliquefaction system according to claim 19.

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