SHIP BOIL-OFF GAS RELIQUEFACTION SYSTEM

Abstract

 Disclosed herein is a boil-off gas reliquefaction system for ships. The boil-off gas reliquefaction system includes: a cargo tank provided to a ship and storing a liquefied gas; a gas reliquefaction line along which boil-off gas generated from the liquefied gas is discharged from the cargo tank and is reliquefied; a compression unit provided to the gas reliquefaction line and receiving the boil-off gas to compress the received boil-off gas; and a condenser provided to the gas reliquefaction line and cooling the boil-off gas compressed through the compression unit, wherein the compression unit includes: a first compressor receiving the boil-off gas and compressing the received boil-off gas; and a second compressor further compressing the boil-off gas compressed by the first compressor and supplying the further compressed boil-off gas to the condenser, and each of the first compressor and the second compressor is provided as a centrifugal compressor.

Description

[Technical Field]

[0001] The present invention relates to a boil-off gas reliquefaction system for ships and, more particularly, to a system for reliquefaction of boil-off gas generated from liquefied gas, in which a centrifugal compressor is used to compress the boil-off gas.

[Background Art]

[0002] 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 onshore or offshore gas pipelines, or transported in a liquid state to distant destinations by liquefied gas carriers. 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 suited to long-distance transportation by sea since the volume thereof is significantly reduced compared to that of natural gas in a gaseous state.

[0003] Conventional LPG carriers employ a fuel supply system that uses heavy fuel oil such as bunker C oil, which is relatively inexpensive, as fuel for propulsion engines. Such a heavy fuel oil supply system has been required to install a separate low sulfur heavy fuel oil (LSHFO) tank due to strengthening of international emission regulations on the use of heavy fuel oil, which has led to great demand for eco-friendly fuel supply systems that meet international environmental standards.

[0004] In recent years, more and more LPG or LNG carriers use a fuel supply system that use LPG, LNG, or boil-off gas generated therefrom as fuel for propulsion. In addition, as international emission regulations tighten, the use of LNG or LPG as fuel for propulsion is also increasing among ships other than LPG or LNG carriers.

[0005] In particular, LPG is easier to store than LNG, which is liquefied at cryogenic temperatures, and has good properties in terms of reduction of SO_X, NOx, CO₂, and PM emissions compared to conventional HFO without being inferior to HFO in terms of specific energy and energy density.

[Disclosure]

[Technical Problem]

[0006] Petroleum gas has a low liquefaction temperature of about -42°C under normal pressure and can be stored in a liquid state up to a temperature of about 45°C at 18 bar and 20°C at 7 bar. Since LPG has a boiling point of -42°C under normal pressure, an LPG storage tank of a ship is insulated. However, due to external heat continuously transferred to the inside of the LPG storage tank, LNG stored in the LNG storage tank continues to evaporate during transportation, causing generation of boil-off gas (BOG).

[0007] Accumulation of boil-off gas in the LPG storage tank can cause excessive increase in pressure in the storage tank, which poses a threat to the safety of the ship and crew. Accordingly, the LPG storage tank has a pressure-resistant structure and a boil-off gas reliquefaction system is used to treat boil-off gas generated in the storage tank.

[0008] The present invention proposes a reliquefaction system that can reliquefy boil-off gas generated from a liquefied gas, such as LPG, and can return the reliquefied boil-off gas to the storage tank, while ensuring increased price competitiveness, reduced footprint, and stable operation.

[Technical Solution]

[0009] In accordance with one aspect of the present invention, there is provided a boil-off gas reliquefaction system for ships, including: a cargo tank provided to a ship and storing a liquefied gas;

a gas reliquefaction line along which boil-off gas generated from the liquefied gas is discharged from the cargo tank and is reliquefied;

a compression unit provided to the gas reliquefaction line and receiving the boil-off gas to compress the boil-off gas; and

a condenser provided to the gas reliquefaction line and cooling the boil-off gas compressed through the compression unit,

wherein the compression unit includes: a first compressor receiving the boil-off gas and compressing the received boil-off gas; and a second compressor further compressing the boil-off gas compressed by the first compressor and supplying the further compressed boil-off gas to the condenser, each of the first compressor and the second compressor being provided as a centrifugal compressor.

[0010] Preferably, the boil-off gas reliquefaction system further includes: a reliquefied gas vessel receiving the boil-off gas cooled and reliquefied by the condenser therein; a pressure transmitter detecting a pressure downstream of the condenser; and a first valve and a second valve disposed in parallel on a vent line along which a vent gas is discharged from the reliquefied gas vessel, wherein a pressure downstream of the second compressor is maintained by controlling the first valve based on the pressure detected by the pressure transmitter and, in an emergency, overpressure in the reliquefied gas vessel is prevented through the second valve.

[0011] Preferably, the boil-off gas reliquefaction system further includes: a reliquefied gas return line connecting the liquefied gas vessel to the cargo tank; and an intercooler provided to the reliquefied gas return line, wherein the boil-off gas compressed in the first compressor is cooled in the intercooler prior to being compressed in the second compressor.

[0012] Preferably, the first compressor is a centrifugal compressor including a first-stage compressor and a second-stage compressor each connected to a motor shaft, wherein the boil-off gas compressed in the first-stage compressor is delivered to the intercooler to be cooled by the reliquefied gas delivered from the reliquefied gas vessel to the cargo tank, is supplied to the second-stage compressor to be compressed, and is supplied to the second compressor to be further compressed.

[0013] Preferably, the boil-off gas reliquefaction system further includes: a knockout drum provided to the gas reliquefaction line upstream of the compression unit and receiving the boil-off gas from the cargo tank to supply a gas to the compression unit.

[Advantageous Effects]

[0014] According to the present invention, a centrifugal compressor is used in a compression unit of a boil-off reliquefaction system in which boil-off gas generated from a liquefied gas stored in a cargo tank is reliquefied through compression and cooling processes and is returned to the cargo tank. By using such a small centrifugal compressor, the system according to the present invention can reduce installation costs compared to when using a reciprocating compressor, thereby increasing price competitiveness, while contributing to securing space in a ship through reduction of footprint. In addition, by using the centrifugal compressor, the system according to the present invention can eliminate the need for additional equipment associated with the use of a reciprocating compressor, such as a dampener, and can prevent increase in maintenance costs due to pulsation phenomena.

[Description of Drawings]

[0015] 

FIG. 1 is a schematic diagram illustrating one example of a system for reliquefying boil-off gas generated from LPG.

FIG. 2 is a schematic diagram of a boil-off gas reliquefaction system for ships according to one embodiment of the present invention.

[Best Mode]

[0016] In order to fully appreciate the operational advantages of the present invention and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings, which illustrate preferred embodiments of the present invention, and description thereof.

[0017] Hereinafter, exemplary embodiments of the present invention will be described in detail in terms of the features and effects thereof with reference to the accompanying drawings. It should be noted that like components will be denoted by like reference numerals throughout the specification and the accompanying drawings.

[0018] In the embodiments of the present invention described herein, the term "ship" may refer to any type of ship. For example, the ship may include self-propelled vessels, such as an LPG carrier, a very large gas carrier (VLGC), an LNG carrier, a liquid hydrogen carrier, and an LNG regasification vessel (RV), as well as non-self-propelled floating offshore structures, such as an LNG floating production storage and offloading (FPSO) unit and an LNG floating storage regasification unit (FSRU).

[0019] In addition, the 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 liquefaction at cryogenic temperatures and can generate boil-off gas during storage. For example, such liquefied gas may include liquefied petrochemical gas, such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas, and liquefied propylene gas, and ammonia. In the following embodiments, the present invention will be described using LPG, which is a typical liquefied gas, as an example.

[0020] FIG. 1 is a schematic diagram illustrating one example of a system for reliquefying boil-off gas generated from LPG.

[0021] Referring to FIG. 1, boil-off gas generated in a cargo tank T is sent to a compressor 20 through a knockout drum 10 to be compressed, is cooled and reliquefied in a condenser 30, and is returned to the cargo tank via a reliquefied gas container 40, an intercooler 50, and the like.

[0022] A typical example of the compressor 20 used to compress boil-off gas in this reliquefaction system is a reciprocating compressor manufactured by Burckhardt Compression AG. However, this reciprocating compressor requires additional equipment such as a dampener due to severe noise and vibration caused by reciprocating pistons while causing increased maintenance costs due to occurrence of pulsation phenomena. In addition, high-price policies of a certain manufacturer using global monopoly supplier status thereof reduces the price competitiveness of ships and makes it difficult to manage delivery schedules due to difficulty in adjusting supply schedules.

[0023] In order to solve these problems, the following embodiments provide a reliquefaction system in which a centrifugal compressor is used to compress boil-off gas, a boil-off gas flow path associated therewith is effectively configured, and a pressure downstream of the compressor is maintained at a level allowing full-reliquefaction of boil-off gas.

[0024] FIG. 2 is a schematic diagram of a boil-off gas reliquefaction system for ships according to one embodiment of the present invention.

[0025] Referring to FIG. 2, the boil-off gas reliquefaction system according to this embodiment includes: a cargo tank T provided in a ship and storing a liquefied gas; a gas reliquefaction line GL along which boil-off gas generated from the liquefied gas is discharged from the cargo tank and is reliquefied; a compression unit 100 provided to the gas reliquefaction line and receiving the boil-off gas to compress the received boil-off gas; and a condenser 200 provided to the gas reliquefaction line and cooling the compressed boil-off gas from the compression unit.

[0026] In addition, the boil-off gas reliquefaction system includes: a knockout drum D provided to the gas reliquefaction line upstream of the compression unit and receiving boil-off gas discharged from the cargo tank to supply a gas to the compression unit. Boil-off gas generated in the cargo tank may include various components such as ethane, propylene, ammonia, etc. in addition to propane and butane (i-butane/n-butane). The boil-off gas generated in the cargo tank is supplied to the knockout drum D along the gas reliquefaction line GL, and the gas separated in the knockout drum D is delivered to the compression unit 100, is reliquefied through a reliquefaction process, and is returned to the cargo tank T.

[0027] The compression unit 100 includes: a first compressor 110 receiving and compressing boil-off gas; and a second compressor 120 further compressing the boil-off gas compressed by the first compressor and supplying the compressed boil-off gas to the condenser, wherein both the first compressor and the second compressor are provided as a centrifugal compressor.

[0028] Specifically, the first compressor 110 is provided as a centrifugal compressor including a first-stage compressor 110A and a second-stage compressor 110B each connected to a motor shaft. However, the number of compression stages in each compressor may be increased as needed. As such, as a centrifugal compressor, which has a lower price, lower weight, and smaller size than a reciprocating compressor, is used in the compression unit, the system according to this embodiment can increase price competitiveness of ships employing the system and can contribute to securing space in a ship while providing ease of placement in the ship through reduction in weight and footprint. In addition, such a centrifugal compressor produces less noise and vibration than a reciprocating compressor with reciprocating pistons and thus can reduce the cost and space required to install additional associated equipment, such as a separate dampener, while ensuring reduced maintenance costs and stable continuous operation through minimization of pulsation phenomena.

[0029] The boil-off gas compressed through three centrifugal compression stages in the first and second compressors is supplied to the condenser 200 to be cooled.

[0030] In the condenser 200, the boil-off gas compressed by the compression unit 100 is cooled and reliquefied through heat exchange. Here, a heat source for cooling the boil-off gas may be, for example, seawater, which is readily available to the ship.

[0031] The boil-off gas cooled and reliquefied in the condenser 200 is delivered to and received in a reliquefied gas vessel 300 and is returned to the cargo tank along a reliquefied gas return line LL connecting the reliquefied gas vessel to the cargo tank T.

[0032] A pressure transmitter PT is disposed downstream of the condenser to detect a pressure downstream of the condenser. A first valve V1 and a second valve V2 are disposed in parallel on a vent line along which a vent gas is discharged from the reliquefied gas vessel. Opening/closing of the first valve V1 and the second valve V2 may be controlled based on the pressure detected by the pressure transmitter.

[0033] Compression pressure required for full liquefaction of boil-off gas varies depending on the composition of the boil-off gas. In the present invention, the pressure downstream of the second compressor 120 is maintained at a level required for full liquefaction of boil-off gas by controlling the first valve V1 based on the pressure detected by the pressure transmitter PT.

[0034] The second valve V2 serves to prevent excessive increase in pressure under emergency conditions. In an emergency, overpressure in the reliquefied gas vessel 300 can be prevented through the second valve V2.

[0035] The reliquefied gas return line LL is provided with an intercooler 400. In the intercooler 400, the boil-off gas compressed by the first compressor 110 undergoes heat exchange with the reliquefied gas to be delivered to the cargo tank T. As shown in FIG. 2, the boil-off gas compressed in the first-stage compressor 110A of the first compressor is delivered to the intercooler 400 to be intermediately cooled by the reliquefied gas delivered from the reliquefied gas vessel 300 to the cargo tank T, is supplied to the second-stage compressor 110B of the first compressor to be compressed, is supplied to the second compressor 120 to be further compressed, and is supplied to the condenser 200 to be cooled and fully reliquefied.

[0036] The boil-off gas (LPG) cooled and reliquefied in the condenser 200 is returned to the cargo tank T via the reliquefied gas vessel 300 and the intercooler 400. By reliquefying boil-off gas and returning the reliquefied boil-off gas to the cargo tank in this manner, the system according to this embodiment allows the pressure in the cargo tank to be maintained at safe levels while increasing the transport rate of LPG.

[0037] Although some embodiments have been described herein, it will be apparent to a person having ordinary knowledge in the art that the present invention is not limited thereto and may be implemented through various modifications or variations without departing from the technical spirit of the present invention.

Claims

1. A boil-off gas reliquefaction system for ships, comprising:

a cargo tank provided to a ship and storing a liquefied gas;

a gas reliquefaction line along which a boil-off gas generated from the liquefied gas is discharged from the cargo tank and is reliquefied;

a compression unit provided to the gas reliquefaction line and receiving the boil-off gas to compress the boil-off gas; and

a condenser provided to the gas reliquefaction line and cooling the boil-off gas compressed through the compression unit,

wherein the compression unit comprises: a first compressor receiving the boil-off gas and compressing the received boil-off gas; and a second compressor further compressing the boil-off gas compressed by the first compressor and supplying the further compressed boil-off gas to the condenser, each of the first compressor and the second compressor being provided as a centrifugal compressor.

2. The boil-off gas reliquefaction system according to claim 1, further comprising:

a reliquefied gas vessel receiving the boil-off gas cooled and reliquefied by the condenser therein;

a pressure transmitter detecting a pressure downstream of the condenser; and

a first valve and a second valve disposed in parallel on a vent line along which a vent gas is discharged from the reliquefied gas vessel,

wherein a pressure downstream of the second compressor is maintained by controlling the first valve based on the pressure detected by the pressure transmitter and, in an emergency, overpressure in the reliquefied gas vessel is prevented through the second valve.

3. The boil-off gas reliquefaction system according to claim 2, further comprising:

a reliquefied gas return line connecting the liquefied gas vessel to the cargo tank; and

an intercooler provided to the reliquefied gas return line,

wherein the boil-off gas compressed in the first compressor is cooled in the intercooler prior to being compressed in the second compressor.

4. The boil-off gas reliquefaction system according to claim 3, wherein the first compressor is a centrifugal compressor comprising a first-stage compressor and a second-stage compressor each connected to a motor shaft, and
the boil-off gas compressed in the first-stage compressor is delivered to the intercooler to be cooled by the reliquefied gas delivered from the reliquefied gas vessel to the cargo tank, is supplied to the second-stage compressor to be compressed, and is supplied to the second compressor to be further compressed.

5. The boil-off gas reliquefaction system according to any one of claims 1 to 4, further comprising:
a knockout drum provided to the gas reliquefaction line upstream of the compression unit and receiving the boil-off gas from the cargo tank to supply a gas to the compression unit.

Drawing