Ship boil-off gas reliquefaction system

Abstract

A ship boil-off gas reliquefaction system is disclosed. A boil-off gas reliquefaction system according to the present invention may comprise: a compression unit that compresses boil-off gas in multiple stages, including a first compressor that receives and compresses the boil-off gas generated from liquefied gas in a cargo tank provided on a ship, and a second compressor that compresses the boil-off gas generated from liquefied gas in a cargo tank provided on the ship; a second compressor for further compressing the boil-off gas compressed in the first compressor; an intercooler receiving and cooling the boil-off gas compressed in the first compressor and then supplying the boil-off gas to the second compressor; a condenser that cools the boil-off gas compressed by the compression unit; a reliquefied gas recovery line that transfers the reliquefied gas cooled and condensed in the condenser to a cargo tank after cold and heat recovery by the intercooler; and a first temperature control line that shunts the liquefied gas from the reliquefied gas recovery line and ejects the liquefied gas onto the top of the intercooler.

Description

Technical Field

[0001] The present invention relates to a ship boil-off gas reliquefaction system, and more particularly to a ship boil-off gas reliquefaction system for recovering boil-off gas generated from liquefied gas in a cargo tank by compressing and reliquefying the boil-off gas. Background Art

[0002] The consumption of liquefied gases such as liquefied natural gas (LNG) and liquefied petroleum gas (LPG) is growing rapidly around the world. Liquefied gases are transported in a gaseous state through land gas pipelines or offshore gas pipelines, or in a liquid state through liquefied gas transport ships to distant destinations. Liquefied gases such as LNG and LPG are obtained by cooling natural gas or petroleum gas to cryogenic temperatures (about -163°C for LNG), and are suitable for long-distance transportation at sea because the volume of liquefied gas is significantly reduced compared to the volume of natural gas in a gaseous state.

[0003] Conventional LPG transport ships adopt a fuel supply system that uses relatively cheap heavy fuel oil, such as marine grade C oil, as fuel for propulsion engines. Due to the strict implementation of international emission regulations for the use of heavy fuel oil, this heavy fuel oil supply system requires a separate low sulfur heavy fuel oil tank (LSHFO tank), which has caused a huge demand for an environmentally friendly fuel supply system that meets international environmental standards.

[0004] In recent years, more and more LPG or LNG transport ships have adopted a fuel supply system that uses LPG, LNG or boil-off gas generated therefrom as fuel for propulsion, and, in response to the strict implementation of international emission regulations, the use of LNG or LPG as fuel for propulsion in ships other than LPG or LNG transport ships is also increasing.

[0005] In particular, LPG is easier to store than LNG which is liquefied at cryogenic temperatures, and has good properties in reducing SOx, NOx, CO2 and PM emissions compared to conventional HFO, while being comparable to HFO in specific energy and energy density. Summary of the invention

[0006] Technical issues

[0007] Under normal pressure, petroleum gas has a low liquefaction temperature of about -42°C, and can be stored in a liquid state at a temperature of up to about 45°C at 18 bar and at a temperature of 20°C at 7 bar. Since LPG has a boiling point of -42°C, the LPG storage tanks of ships are insulated. However, since external heat is continuously transferred to the LPG storage tanks, the LNG stored in the LNG storage tanks continuously volatilizes during transportation, resulting in the generation of boil-off gas.

[0008] The accumulation of boil-off gas in LPG storage tanks can cause excessive increase in the internal pressure of the storage tanks, which poses a threat to the safety of the ship and crew. Therefore, LPG storage tanks have a pressure-resistant structure and a boil-off gas reliquefaction system is used to process the boil-off gas generated in the storage tanks.

[0009] The present invention provides a reliquefaction system that can reliquefy boil-off gas generated from liquefied gas such as LPG, and can return the reliquefied boil-off gas to a storage tank to adjust the tank pressure while reducing the number of necessary facilities, thereby reducing costs and improving the price competitiveness of ships.

[0010] Technical Solutions

[0011] According to one aspect of the present invention, a boil-off gas reliquefaction system for a ship is provided, the boil-off gas reliquefaction system comprising: a compression unit, comprising a first compressor and a second compressor, the first compressor receiving boil-off gas generated by liquefied gas in a cargo tank of the ship and compressing the boil-off gas, the second compressor further compressing the boil-off gas compressed in the first compressor and performing multi-stage compression on the boil-off gas;

[0012] an intercooler that supplies boil-off gas compressed in the first compressor to the second compressor after cooling the compressed boil-off gas received from the first compressor;

[0013] A condenser for cooling the boil-off gas compressed by the compression section;

[0014] a reliquefied gas recovery line for transferring the liquefied gas cooled and condensed in the condenser to the cargo tank after recovering cold heat from the liquefied gas through the intercooler; and

[0015] The first temperature regulating line branches off the liquefied gas from the reliquefied gas recovery line and sprays the liquefied gas onto the top of the intercooler.

[0016] Preferably, the boil-off gas reliquefaction system further comprises: a first temperature sensor for sensing the temperature of the boil-off gas to be delivered to the second compressor after being cooled in the intercooler; and a first temperature regulating valve for regulating the flow rate of the liquefied gas to be diverted into the first temperature regulating line, wherein the first temperature regulating valve is controlled according to the temperature of the boil-off gas sensed by the first temperature sensor to regulate the temperature of the boil-off gas delivered to the second compressor.

[0017] Preferably, the boil-off gas reliquefaction system also includes: a liquid level regulating pipeline, which branches off from the reliquefied gas recovery pipeline and transports the liquefied gas to the intercooler; a liquid level sensor, which senses the liquid level of the liquefied gas inside the intercooler; and a liquid level regulating valve, which opens or closes the liquid level regulating pipeline, wherein the liquid level regulating valve is controlled according to the liquid level of the liquefied gas sensed by the liquid level sensor to maintain the liquid level of the liquefied gas inside the intercooler within a certain range, and the boil-off gas compressed in the first compressor is supplied to the liquefied gas in the intercooler through a spray nozzle to be cooled by the liquefied gas.

[0018] Preferably, the boil-off gas reliquefaction system further comprises: a knockout drum receiving boil-off gas discharged from the cargo tank to supply the boil-off gas to the first compressor of the compression section; and a second temperature regulating line branching off from the reliquefied gas recovery line downstream of the intercooler and connected to the knockout drum, wherein the liquefied gas in the reliquefied gas recovery line is conveyed to the lower side of the knockout drum to cool the boil-off gas to be conveyed to the compression section.

[0019] Preferably, the boil-off gas reliquefaction system further comprises: a second temperature sensor for sensing the temperature of the boil-off gas compressed in the first compressor and introduced into the intercooler; and a second temperature regulating valve for regulating the flow rate of the liquefied gas to be diverted into the second temperature regulating pipeline, wherein the second temperature regulating valve is controlled according to the temperature of the boil-off gas sensed by the second temperature sensor to regulate the flow rate of the liquefied gas to be transported to the lower side of the separation drum.

[0020] Preferably, the boil-off gas reliquefaction system further comprises: a reliquefied gas receiver provided in the reliquefied gas recovery line upstream of the intercooler to receive the reliquefied gas cooled in the condenser, wherein the gas is separated in the reliquefied gas receiver to be discharged to the exhaust line.

[0021] Preferably, the liquefied gas includes liquefied petroleum gas (LPG), and the compression part is a three-stage centrifugal compressor or a three-stage reciprocating compressor.

[0022] Beneficial effects

[0023] In the boil-off gas reliquefaction system according to the present invention, when the reliquefied gas is returned to the cargo tank after the boil-off gas generated from the liquefied gas stored in the cargo tank is reliquefied by cooling and compressing the boil-off gas, the reliquefaction rate of the boil-off gas can be improved by maximizing the cooling effect of the intercooler while reducing the power consumption of the compression section, taking into account that the multi-component boil-off gas allows large changes in physical properties.

[0024] Furthermore, the boil-off gas reliquefaction system according to the present invention makes it possible to reduce the number of necessary facilities on a ship, thereby reducing installation costs while improving the price competitiveness of the ship, and enables boil-off gas to be returned to the cargo tank by reliquefaction, thereby improving the LPG transportation rate while maintaining the internal pressure of the cargo tank. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic diagram showing an example of a system for reliquefying boil-off gas generated from LPG.

[0026] Figure 2 Schematic diagram of a ship boil-off gas reliquefaction system according to an embodiment of the present invention.

[0027] Figure 3 FIG. 4 is a schematic diagram of a ship boil-off gas reliquefaction system according to another embodiment of the present invention. DETAILED DESCRIPTION

[0028] For a fuller understanding of the operating advantages of the invention and the objectives attained by its practice, reference should be made to the accompanying drawings and descriptive notes in which preferred embodiments of the invention are illustrated.

[0029] Hereinafter, exemplary embodiments of the present invention will be described in detail in terms of the features and effects of the present invention with reference to the accompanying drawings. It should be noted that the same components will be represented by the same reference numerals throughout the specification and all the drawings.

[0030] In the embodiments of the present invention described herein, the term "vessel" may refer to any type of vessel. For example, the vessel may include: a self-propelled vessel, such as an LPG carrier, a very large gas carrier (VLGC), an LNG carrier, a liquid hydrogen carrier, and an LNG regasification vessel (RV); and a non-self-propelled floating offshore structure, such as an LNG floating production storage and offloading (FPSO) unit and an LNG floating storage regasification unit (FSRU).

[0031] In addition, embodiments of the present invention may be applied to a reliquefaction system for any type of liquefied gas that can be transported in a liquid state by liquefying at cryogenic temperatures and that can generate boil-off gas during storage. For example, such liquefied gas may include liquefied petrochemical gases such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas, and liquefied propylene gas, as well as ammonia. The following description of embodiments of the present invention will focus on the application of LPG as the liquefied gas by way of example.

[0032] Figure 1 is a schematic diagram showing an example of a system for reliquefying boil-off gas generated from LPG.

[0033] Reference Figure 1 , boil-off gas generated in the cargo tank is sent to the compressor 20 through the separator drum 10 to be compressed, is cooled and reliquefied in the condenser 30, and is returned to the cargo tank via the reliquefied gas container 60, the intercooler 50 and the like.

[0034] In order to compress the boil-off gas to the reliquefaction pressure, the compressor 20 may include a multi-stage compressor, such as a three-stage compressor, and is generally operated at a constant speed. In order to improve the operating efficiency of the compressor, the boil-off gas compressed in the first stage compressor is cooled by an intercooler before the boil-off gas is introduced into the second stage compressor, and the intercooler can utilize the cold heat of the reliquefied LPG.

[0035] However, boil-off gas generated from LPG in a cargo tank contains various components such as ethane, propylene, ammonia, and the like in addition to propane and butane (isobutane (I-Butane) / normal butane (N-Butane)), and is allowed to greatly change in physical properties.

[0036] In order to improve the compressor efficiency and reliquefaction efficiency by effectively cooling the multi-component boil-off gas and thereby allowing a large change in physical properties, a trim cooler 40 is provided to the system to perform additional cooling on the boil-off gas to be introduced into the second-stage compressor after being cooled in the intercooler, and a separate intercooler may be further installed between the second-stage compressor and the third-stage compressor.

[0037] However, due to the characteristics of the compressor running at a constant rate and the large variation in the amount of multi-component boil-off gas, it is difficult to effectively control the temperature of the boil-off gas introduced into the second-stage compressor through the intercooler and the temperature-adjusting cooler, and when the boil-off gas is introduced into the compressor without being sufficiently cooled, there are problems of increased power consumption of the compressor and reduced final liquefaction efficiency. Therefore, complete reliquefaction of the boil-off gas is difficult in practice, and the non-liquefied gas is processed through a separate exhaust gas system. The reliquefaction system according to an embodiment of the present invention is designed to solve these problems, thereby achieving a reduction in power consumption of the compressor and equipment cost while improving the reliquefaction efficiency by effectively cooling the multi-component boil-off gas and thereby allowing large changes in physical properties.

[0038] Figure 2 is a schematic diagram of a ship boil-off gas reliquefaction system according to an embodiment of the present invention, and Figure 3 FIG. 4 is a schematic diagram of a ship boil-off gas reliquefaction system according to another embodiment of the present invention.

[0039] Reference Figure 2According to this embodiment, the boil-off gas reliquefaction system includes: a compression section 100, which performs multi-stage compression of boil-off gas, and includes: a first compressor 100A, which receives boil-off gas generated from liquefied gas in a cargo tank on a ship and compresses the boil-off gas; and a second compressor 100B, which additionally compresses the boil-off gas compressed in the first compressor; an intercooler 200, which receives compressed boil-off gas from the first compressor and supplies the compressed boil-off gas to the second compressor after cooling the compressed boil-off gas; a condenser 300, which cools the boil-off gas compressed by the compression section; a reliquefied gas recovery pipeline RL, which transports the reliquefied boil-off gas cooled and condensed in the condenser to the cargo tank after recovering cold and heat from the reliquefied boil-off gas through the intercooler; and a first temperature regulating pipeline TL1, which diverts the liquefied gas from the reliquefied gas recovery pipeline and sprays the liquefied gas onto the top of the intercooler.

[0040] The boil-off gas reliquefaction system includes: a separation drum 500, a gas reliquefaction line GL provided to the upstream of the compression section, and receiving boil-off gas discharged from the cargo tank to supply the gas to the compression section 100. The boil-off gas generated in the cargo tank may contain various components such as ethane, propylene, ammonia, and the like in addition to propane and butane (isobutane / normal-butane). The multi-component boil-off gas generated in the cargo tank is discharged to the separation drum 500 along the gas reliquefaction line GL, and the gas separated in the separation drum is transported to the first compressor of the compression section 100 to undergo a reliquefaction process, and finally returned to the cargo tank.

[0041] The compression part 100 may be a three-stage compressor including a first compressor 100A that receives and compresses boil-off gas, a second compressor 100B that further compresses the boil-off gas compressed in the first compressor, and a third compressor 100C that further compresses the boil-off gas compressed in the second compressor and delivers the compressed boil-off gas to a condenser. The compression part may be composed of a three-stage centrifugal compressor including the first compressor to the third compressor, or may be composed of a three-stage reciprocating compressor having a reciprocating piston, such as Figure 3 As shown in the embodiment shown. Additional compression stages can be added as needed.

[0042] The boil-off gas compressed by the compression part is supplied to the condenser 300 , and the boil-off gas is cooled in the condenser 300 .

[0043] In the condenser 300, the boil-off gas compressed by the compression part 100 is cooled to reliquefy the boil-off gas through heat exchange, and a heat source for cooling the boil-off gas may include, for example, seawater that can be easily obtained from a ship.

[0044] The reliquefied gas receiver 400 receives the reliquefied gas cooled in the condenser 300 along the reliquefied gas recovery line, the liquid in the reliquefied gas receiver is returned to the cargo tank along the reliquefied gas recovery line RL, and the exhaust gas separated in the reliquefied gas receiver is discharged through the exhaust line.

[0045] The reliquefied gas recovery line RL is connected to the cargo tank through the intercooler 200. In the intercooler 200, the boil-off gas compressed in the first compressor 100A is heat-exchanged with the reliquefied gas to be delivered to the cargo tank. Figure 2 As shown, the boil-off gas compressed in the first compressor 100A of the compression section is sequentially delivered to the intercooler 200 to be intercooled by the reliquefied gas delivered from the reliquefied gas receiver 400 to the cargo tank, and then delivered to the second compressor 100B to be compressed in the second compressor 100B, and then delivered to the third compressor 100C to be further compressed in the third compressor 100C, and delivered to the condenser 300 to be cooled and reliquefied.

[0046] The boil-off gas reliquefaction system is further provided with: a liquid level regulating pipeline LL, branching from the reliquefied gas recovery pipeline RL, to deliver the liquefied gas to the intercooler 200; a liquid level sensor LIC, sensing the level of the liquefied gas inside the intercooler; and a liquid level regulating valve LV, opening or closing the liquid level regulating pipeline. According to the liquid level sensed by the liquid level sensor, the liquid level controller LIC controls the liquid level regulating valve to maintain the level of the liquefied gas inside the intercooler within a certain range.

[0047] The boil-off gas compressed in the first compressor 100A may be supplied through a spray nozzle to the liquefied gas in the intercooler 200. The liquefied gas passing through the reliquefied gas recovery line via the reliquefied gas receiver may also supply cold heat to the intercooler while passing through a heat pipe provided below the intercooler and filled with liquefied gas, and the boil-off gas compressed in the first compressor is cooled in the intercooler.

[0048] Furthermore, in this embodiment, taking into account that the multi-component boil-off gas is allowed to undergo large changes in physical properties, the boil-off gas reliquefaction system is also provided with features for maximizing the cooling effect and finely adjusting the temperature of the boil-off gas to be delivered to the second compressor, so as to increase the reliquefaction rate while reducing compression power consumption.

[0049] To this end, the boil-off gas reliquefaction system according to this embodiment includes: a first temperature regulating line TL1, which diverts liquefied gas from the reliquefied gas recovery line RL and sprays the liquefied gas onto the top of the intercooler 200; a first temperature sensor TIT1, which detects the temperature of the boil-off gas cooled in the intercooler to be delivered to the second compressor; and a first temperature regulating valve TV1, which regulates the flow rate of the liquefied gas diverted to the first temperature regulating line.

[0050] According to the temperature of the boil-off gas downstream of the intercooler sensed by the first temperature sensor TIT1, the first temperature controller TI1 controls the first temperature regulating valve TV1 so that a portion of the liquefied gas in the reliquefied gas recovery line is diverted to the first temperature regulating line to be sprayed onto the top of the intercooler, so that the boil-off gas cooled at the lower side of the intercooler can be further cooled, thereby adjusting the temperature of the boil-off gas introduced into the second compressor. The first temperature regulating line may be provided at its distal end with a spray nozzle for additional cooling to spray the liquefied gas.

[0051] In this way, since the intercooler improves the cooling efficiency of the evaporated gas to be introduced into the second compressor after being compressed in the first compressor of the compression section, and at the same time enables fine adjustment of the temperature of the evaporated gas upstream of the second compressor, even in the absence of a separate intercooling device between the second compressor and the third compressor, the equipment cost and the power consumption of the compression section can be reduced while improving the reliquefaction efficiency.

[0052] Furthermore, the boil-off gas is reliquefied by the boil-off gas reliquefaction system according to this embodiment and returned to the cargo tank, thereby improving the LPG transportation rate while safely maintaining the temperature in the cargo tank.

[0053] according to Figure 3 The boil-off gas reliquefaction system of another embodiment shown is configured to increase the reliquefaction rate while further reducing the power consumption of the compression section by additionally reducing the temperature of the boil-off gas to be delivered from the separation drum to the first compressor of the compression section.

[0054] For this purpose, refer to Figure 3 The boil-off gas reliquefaction system according to this embodiment may further include a second temperature regulating line TL2, which branches off from the reliquefied gas recovery line RL downstream of the intercooler 200 and is connected to the separation drum, so as to cool the boil-off gas to be transported to the compression part 100 by spraying a portion of the liquefied gas in the reliquefied gas recovery line into the separation drum 500.

[0055] The boil-off gas reliquefaction system according to this embodiment further includes a second temperature sensor TIT2 and a second temperature regulating valve TV2, wherein the second temperature sensor TIT2 detects the temperature of the boil-off gas compressed by the first compressor and introduced into the intercooler, and the second temperature regulating valve TV2 regulates the flow rate of the liquefied gas to be diverted into the second temperature regulating line. The liquefied gas returned to the cargo tank through the reliquefied gas recovery line may be in a sub-cooled state. By controlling the second temperature regulating valve according to the temperature of the boil-off gas downstream of the first compressor sensed by the second temperature sensor, a portion of the liquefied gas in a sub-cooled state downstream of the reliquefied gas recovery line may be diverted into the second temperature regulating line and conveyed to the lower side of the separation drum to cool the boil-off gas, and the temperature of the boil-off gas introduced into the first compressor of the compression section may be regulated.

[0056] Repetitive description of the same features as those of the system according to the above-described embodiment is omitted.

[0057] In this way, a portion of the cold liquefied gas downstream of the reliquefied gas recovery pipeline is diverted into the second temperature regulating pipeline and transported to the separation drum to cool the boil-off gas to be transported to the compression section, and a portion of the cold liquefied gas is sprayed toward the upper portion of the intercooler through the first temperature regulating pipeline so that the temperature of the boil-off gas to be transported to the second compressor can be finely adjusted, thereby reducing the power consumption of the compression section while improving the final reliquefaction rate.

[0058] Although some embodiments have been described herein, it is obvious to those skilled in the art that the present invention is not limited thereto and may be implemented through various modifications or changes without departing from the technical spirit of the present invention.

Claims

1. A ship boil-off gas reliquefaction system, comprising: a compression unit, comprising a first compressor and a second compressor, wherein the first compressor receives boil-off gas generated by liquefied gas in a cargo tank installed in the ship and compresses the boil-off gas, and the second compressor further compresses the boil-off gas compressed in the first compressor and performs multi-stage compression on the boil-off gas; an intercooler that supplies the boil-off gas compressed in the first compressor to the second compressor after cooling the compressed boil-off gas received from the first compressor; a condenser for cooling the boil-off gas compressed by the compression unit; through a reliquefied gas recovery line, after recovering cold heat from the liquefied gas through the intercooler, the liquefied gas cooled and condensed in the condenser is transferred to the cargo tank; and A first temperature regulating line branches off the liquefied gas from the reliquefied gas recovery line and sprays the liquefied gas onto the top of the intercooler.

2. The ship boil-off gas reliquefaction system according to claim 1, further comprising: a first temperature sensor to sense a temperature of the boil-off gas to be delivered to the second compressor after being cooled in the intercooler; as well as a first temperature regulating valve for regulating the flow rate of the liquefied gas to be diverted into the first temperature regulating pipeline, The first temperature regulating valve is controlled according to the temperature of the boil-off gas sensed by the first temperature sensor to adjust the temperature of the boil-off gas delivered to the second compressor.

3. The ship boil-off gas reliquefaction system according to claim 2, further comprising: a liquid level regulating line branching from the reliquefied gas recovery line and conveying the liquefied gas to the intercooler; a liquid level sensor to sense the liquid level of the liquefied gas inside the intercooler; and A liquid level regulating valve opens or closes the liquid level regulating pipeline, wherein the liquid level regulating valve is controlled according to the liquid level of the liquefied gas sensed by the liquid level sensor to maintain the liquid level of the liquefied gas inside the intercooler within a certain range, and The boil-off gas compressed in the first compressor is supplied to the liquefied gas in the intercooler through a spray nozzle to be cooled by the liquefied gas.

4. The ship boil-off gas reliquefaction system according to claim 2, further comprising: a separation cylinder receiving the boil-off gas exhausted from the cargo tank to supply the boil-off gas to the first compressor of the compression section; as well as a second temperature regulating line branched from the reliquefied gas recovery line downstream of the intercooler and connected to the separation drum, The liquefied gas in the reliquefied gas recovery line is transported to the lower side of the separation drum to cool the boil-off gas to be transported to the compression part.

5. The ship boil-off gas reliquefaction system according to claim 4, further comprising: a second temperature sensor to sense a temperature of the boil-off gas compressed in the first compressor and introduced into the intercooler; as well as a second temperature regulating valve for regulating the flow rate of the liquefied gas to be diverted into the second temperature regulating pipeline, The second temperature regulating valve is controlled according to the temperature of the boil-off gas sensed by the second temperature sensor to adjust the flow rate of the liquefied gas to be delivered to the lower side of the separation drum.

6. The ship boil-off gas reliquefaction system according to any one of claims 1 to 5, further comprising: a reliquefied gas receiver disposed in the reliquefied gas recovery line upstream of the intercooler to receive the reliquefied gas cooled in the condenser, The gas is separated in the reliquefied gas receiver to be discharged to a gas exhaust line. 7 . The ship boil-off gas reliquefaction system according to claim 6 , wherein the liquefied gas comprises liquefied petroleum gas (LPG), and the compression section is a three-stage centrifugal compressor or a three-stage reciprocating compressor.

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