Patent Application
20250020399
The present invention relates to a blow-down method for a reliquefaction system for ships and, more particularly, to a blow-down method for a reliquefaction system for ships, which can prevent damage to a heat exchanger in the event of a trip of a reliquefaction system for reliquefying boil-off gas generated in a ship by promptly discharging compressed gas and reliquefied gas trapped in the heat exchanger and a reliquefaction line.
Natural gas contains methane as a main component and has been attracting attention as an eco-friendly fuel that emits little or no environmental pollutants during combustion. Liquefied natural gas (LNG) is obtained by liquefying natural gas through cooling to about −163° C. under normal pressure and is suited to long-distance transportation by sea since it has a volume of about 1/600 that of natural gas in a gaseous state. Accordingly, natural gas is stored and transported as liquefied natural gas, which is easy to store and transport.
Since natural gas is liquefied at a cryogenic temperature of −163° C. under normal pressure, LNG storage tanks are typically insulated to maintain LNG in a liquid state. However, despite being insulated, such a storage tank is limited in ability to block external heat. Accordingly, due to external heat continuously transferred to the LNG storage tank, LNG stored in the LNG tank continues to evaporate naturally during transportation, causing generation of boil-off gas (BOG).
Continuous production of boil-off gas in the LNG storage tank increases the internal pressure of the LNG storage tank. If the internal pressure of the storage tank exceeds a predetermined safe pressure, this can cause an emergency situation such as rupture of the storage tank. Accordingly, there is a need to discharge boil-off gas from the storage tank using a safety valve. However, boil-off gas is a kind of LNG loss and is an important issue for transportation efficiency and fuel efficiency of LNG. Therefore, various methods are employed to handle boil-off gas generated in the LNG storage tank.
Recently, a method of using boil-off gas at a fuel demand site such as an engine of a ship, a method of reliquefying boil-off gas and returning the reliquefied boil-off gas to an LNG storage tank, and a method combining these two approaches have been developed and put into use.
In a reliquefaction cycle for reliquefaction of boil-off gas generated in a ship, typical available liquefaction methods include a process using a single mixed refrigerant (SMR) cycle and a process using a propane-precooled mixed refrigerant (C3MR) cycle. The C3MR cycle is a process in which natural gas is cooled using propane refrigerant alone and then is liquefied and subcooled using a mixed refrigerant, while the SMR cycle is a process in which natural gas is liquefied using a mixed refrigerant composed of multiple components.
As such, the SMR cycle and the C3MR cycle both use a mixed refrigerant. However, if the composition of the mixed refrigerant changes due to refrigerant loss during liquefaction of boil-off gas, this can lead to poor liquefaction efficiency. Accordingly, there is a need to maintain constant composition of the refrigerant by continuously measuring the composition of the mixed refrigerant and replenishing lacking refrigerant components.
An alternative reliquefaction cycle to reliquefy boil-off gas is a single-cycle liquefaction process using nitrogen refrigerant.
Despite relative inefficiency compared to a reliquefaction cycle using a mixed refrigerant, such a reliquefaction cycle using nitrogen refrigerant is safer due to inert properties of nitrogen refrigerant and is easier to apply to ships since nitrogen refrigerant undergoes no phase change.
A reliquefaction system includes: a compressor receiving and compressing boil-off gas; a heat exchanger in which the compressed gas from the compressor is cooled through heat exchange with a refrigerant; and a refrigerant circulation unit through which the refrigerant exchanging heat with the compressed gas in the heat exchanger is circulated. When the reliquefaction system employs a refrigeration cycle using nitrogen refrigerant, the refrigerant circulation unit may be configured such that the refrigerant discharged from the heat exchanger after exchanging heat with the compressed gas is compressed, cooled through the heat exchanger, expanded and cooled again, and circulated to the heat exchanger.
However, if the fluids entering and exiting the heat exchanger are not controlled promptly in the event of abnormal operation of the reliquefaction system, the heat exchanger can be subjected to significant thermal stresses, which can lead to damage to the heat exchanger, such as fatigue failure, and reduction in lifespan of the heat exchanger.
To solve this problem, the present invention proposes a method that can prevent damage to the heat exchanger in the event of abnormal operation of the reliquefaction system by allowing the fluids entering and exiting the heat exchanger to be controlled promptly.
In accordance with one aspect of the present invention, there is provided a blow-down method of a reliquefaction system for ships, wherein boil-off generated from a liquefied gas stored in a storage tank of a ship is compressed by a compressor, cooled and reliquefied through a heat exchanger along a reliquefaction line, and returned to the storage tank after passing through a gas-liquid separator, and
Preferably, the reliquefaction system includes: a bypass line connecting a downstream side of the heat exchanger to the storage tank without passing through the gas-liquid separator; a first valve disposed on the reliquefaction line downstream of a junction between the reliquefaction line and the bypass line; and a second valve disposed on the bypass line, wherein the compressed gas and the reliquefied gas removed from the heat exchanger and the reliquefaction line, and the nitrogen are discharged along the bypass line.
Preferably, the reliquefaction system further includes: a third valve disposed on the pressure compensation line upstream of a junction between the pressure compensation line and the nitrogen blanket line; a fourth valve disposed on the nitrogen blanket line; and a fifth valve disposed on the pressure compensation line downstream of the junction between the pressure compensation line and the nitrogen blanket line.
Preferably, a refrigerant circulated along a refrigerant circulation line is supplied to the heat exchanger to cool the compressed gas, and uncompressed boil-off gas from the storage tank is supplied to the compressor after passing through the heat exchanger such that the compressed gas is cooled in the heat exchanger through heat exchange with the refrigerant and the uncompressed boil-off gas.
Preferably, the refrigerant circulated along the refrigerant circulation line is nitrogen and the heat exchanger is a brazed aluminum heat exchanger (BAHE).
Preferably, upon delivery of the reliquefied gas from the gas-liquid separator
to the storage tank, a pressure in the gas-liquid separator is maintained by flash gas generated from the reliquefied gas supplied to the gas-liquid separator, and, when the reliquefied gas supplied to the gas-liquid separator is in a subcooled state and fails to generate enough flash gas to maintain the pressure in the gas-liquid separator, boil-off gas or nitrogen is supplied to the gas-liquid separator along the pressure compensation line to maintain the pressure in the gas-liquid separator.
The blow-down method according to the present invention can reduce thermal fatigue of a heat exchanger and can prevent damage to the heat exchanger in the event of abnormal operation of a reliquefaction system, for example, in the event of a trip of the reliquefaction system, by promptly discharging compressed gas and reliquefied gas trapped in the heat exchanger and a reliquefaction line while purging the heat exchanger and the reliquefaction line with nitrogen.
In addition, the blow-down method according to the present invention can reduce installation costs and footprint by utilizing existing equipment without requiring separate equipment and can provide reduced maintenance costs and increased safety of a ship by preventing damage to the heat exchanger and pipelines connected thereto.
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.
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.
As used herein, the term “ship” may refer to any type of ship that is provided with a liquefied gas storage tank. For example, the ship may include self-propelled vessels, such as 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).
In addition, the embodiments of the present invention may be applied to a reliquefaction cycle 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. In the following embodiments, the present invention will be described using LNG, which is a typical liquefied gas, as an example.
A reliquefaction system for ships according to this embodiment is a system in which boil-off gas generated from a liquefied gas stored in a storage tank of a ship is discharged through a vapor header, is delivered to a compressor to be compressed, and is supplied as fuel to an engine of the ship or the like, as needed, and surplus boil-off gas is delivered to a heat exchanger 100 along a reliquefaction line RL to be cooled and reliquefied through heat exchange and is returned to the storage tank.
In the reliquefaction system, the boil-off gas generated from the liquefied gas stored in the storage tank of the ship is discharged through the vapor header (not shown) and is supplied to the compressor (not shown) along a gas supply line (not shown), wherein the gas supply line is connected from the storage tank to the compressor through the heat exchanger such that uncompressed boil-off gas from the storage tank supplies cold heat to the heat exchanger.
The compressed boil-off gas from the compressor is introduced back into the heat exchanger and is cooled by cold heat from the uncompressed boil-off gas flowing through the gas supply line.
In addition to the uncompressed boil-off gas, the heat exchanger 100 may be supplied with a separate refrigerant circulated along a refrigerant circulation line. The refrigerant circulated along the refrigerant circulation line may be nitrogen, and the refrigerant circulation line is provided with a refrigerant compressor compressing the nitrogen refrigerant and a refrigerant expander. The nitrogen refrigerant is compressed by the refrigerant compressor, is cooled through the heat exchanger, is expanded and cooled again by the refrigerant expander, and is supplied as refrigerant to the heat exchanger while circulating along the refrigerant circulation line. Accordingly, in the heat exchanger, four different streams, that is, the compressed boil-off gas from the compressor, the uncompressed boil-off gas to be supplied to the compressor, the expanded and cooled refrigerant from the refrigerant expander, and the compressed refrigerant from the refrigerant compressor, participate in heat exchange.
The heat exchanger may be a cryogenic heat exchanger, for example, a brazed aluminum heat exchanger (BAHE), which is suitable for the refrigeration cycle using nitrogen and boil-off gas generated from LNG at extremely low temperatures.
The cooled boil-off gas from the heat exchanger 100 is delivered to a gas-liquid separator 200 to be subjected to gas-liquid separation, and the separated reliquefied gas is returned to the storage tank CH. The system according to this embodiment may include a bypass line BL branched off of the reliquefaction line RL downstream of the heat exchanger and connected to the storage tank without passing through the gas-liquid separator, such that the cooled reliquefied gas from the heat exchanger can be delivered directly to the storage tank. The reliquefaction line RL is provided with a first valve V1 downstream of a junction between the reliquefaction line and the bypass line, and the bypass line BL is provided with a second valve V2 to open/close the bypass line.
Opening a valve LV downstream of the gas-liquid separator to deliver the separated reliquefied gas from the gas-liquid separator to the storage tank can cause changes in pressure in the gas-liquid separator. Here, the pressure in the gas-liquid separator can be kept constant by flash gas generated from liquefied gas supplied to the separator, that is, off-gas.
However, if the liquefied gas to be supplied to the gas-liquid separator is cooled to a subcooled state through heat exchange with the nitrogen refrigerant in the heat exchanger, this can result in little or no off-gas being generated from the liquefied gas and thus opening of the valve downstream of the gas-liquid separator can cause a sudden drop in pressure in the gas-liquid separator. In order to compensate for such pressure variations in the gas-liquid separator to maintain a constant pressure in the gas-liquid separator, the system according to this embodiment includes: a pressure compensation line PL branched off of the reliquefaction line RL downstream of the compressor and connected to an upside of the gas-liquid separator 200; and a nitrogen blanket line NBL along which nitrogen is supplied to the pressure compensation line. In this way, the pressure in the gas-liquid separator can be kept constant by supplying boil-off gas or nitrogen to the gas-liquid separator along the pressure compensation line PL upon delivery of reliquefied gas from the gas-liquid separator to the storage tank.
A third valve V3 is disposed on the pressure compensation line PL upstream of a junction between the pressure compensation line PL and the nitrogen blanket line NBL, a fourth valve V4 is disposed on the nitrogen blanket line NBL, and a fifth valve V5 is disposed on the pressure compensation line PL downstream of the junction between the pressure compensation line PL and the nitrogen blanket line NBL.
Referring to
The temperature of boil-off gas generated in the storage tank and supplied to the heat exchanger is usually about −100° C. and, depending on the condition of the storage tank, boil-off gas at a temperature of −130° C. or less can be generated in the storage tank. When there is a sudden change in temperature of boil-off gas, or when there is a large difference in temperature between the heat exchanger and the boil-off gas, such as at the beginning of startup of the reliquefaction system, the heat exchanger can be subjected to significant thermal stress. In particular, if the fluids entering and leaving the heat exchanger are not controlled promptly in the event of abnormal operation of the reliquefaction system, thermal stresses on the heat exchanger can increase significantly, causing damage to the heat exchanger, such as fatigue failure, and reduction in lifespan of the heat exchanger. In order to prevent such damage to the heat exchanger, the blow-down method according to this embodiment allows compressed gas and reliquefied gas remaining in the heat exchanger and pipelines connected to the heat exchanger to be discharged promptly in the event of abnormal operation of the reliquefaction system.
The blow-down method according to this embodiment allows compressed gas and reliquefied gas trapped in the heat exchanger and the reliquefaction line to be discharged therefrom while purging the heat exchanger and the reliquefaction line with nitrogen by utilizing the pressure compensation line PL and the nitrogen blanket line NBL, which are provided to maintain a constant pressure in the gas-liquid separator.
That is, in the event of a trip of the reliquefaction system, nitrogen is supplied to the pressure compensation line PL along the nitrogen blanket line NBL by opening the fourth valve V4 and the third valve V3 and is delivered to a downstream side of the compressor to pass through the heat exchanger 100 along the reliquefaction line RL. In addition, the first valve V1 upstream of the gas-liquid separator is closed and the second valve V2 on the bypass line BL is opened to discharge compressed gas and reliquefied gas from the heat exchanger and the reliquefaction line along the bypass while subjecting the heat exchanger and the reliquefaction line to nitrogen (N2) purging.
As such, by promptly discharging compressed gas and reliquefied gas trapped in the heat exchanger and the reliquefaction line while performing nitrogen purging in the event of abnormal operation of the reliquefaction system, it is possible to reduce thermal stress on the heat exchanger, which is sensitive to temperature changes, thereby preventing thermal fatigue of the heat exchanger and damage to the heat exchanger.
In particular, the blow-down method according to the present invention can reduce installation costs and footprint by utilizing existing equipment without requiring separate equipment and can provide reduced maintenance costs and increased safety of a ship by preventing damage to the heat exchanger and pipelines connected thereto.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2021-0145490 | Oct 2021 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2021/019906 | 12/27/2021 | WO |
