EP-4741278-A1 - BOIL-OFF GAS TREATMENT SYSTEM

EP4741278A1EP 4741278 A1EP4741278 A1EP 4741278A1EP-4741278-A1

Abstract

The present invention relates to a boil-off gas treatment system. A boil-off gas treatment system according to the present invention includes: a storage unit in which a liquefied gas is stored; a subcooling unit connected to the storage unit and configured to subcool boil-off gas generated from the liquefied gas stored in the storage unit to generate subcooled boil-off gas; a compression unit connected to the subcooling unit and configured to receive the subcooled boil-off gas from the subcooling unit and compress the subcooled boil-off gas to generate compressed subcooled boil-off gas; and a phase change unit connected to the compression unit, configured to receive the compressed subcooled boil-off gas from the compression unit, and configured to induce a phase change of the compressed subcooled boil-off gas into a liquid/solid-based slush state.

Inventors

LIM, JONG WOONG
PARK, SUNG HO
CHUNG, SOH MYUNG
CHOI, KWANG SOON
LEE, CHANG HYEONG
PARK, JUN SEOK
YOON, SANG HEE
YU, JU YOUNG

Assignees

Institute for Advanced Engineering

Dates

Publication Date: 20260513
Application Date: 20240805

Claims (7)

A boil-off gas treatment system, comprising: a storage unit in which a liquefied gas is stored; a subcooling unit connected to the storage unit and configured to subcool boil-off gas generated from the liquefied gas stored in the storage unit to generate subcooled boil-off gas; a compression unit connected to the subcooling unit and configured to receive the subcooled boil-off gas from the subcooling unit and compress the subcooled boil-off gas to generate compressed subcooled boil-off gas; and a phase change unit connected to the compression unit and configured to receive the compressed subcooled boil-off gas from the compression unit and induce a phase change of the compressed subcooled boil-off gas into a liquid/solid-based slush state.
The boil-off gas treatment system of claim 1, wherein the phase change unit includes: a nozzle unit connected to the compression unit and configured to provide a flow path that induces the compressed subcooled boil-off gas to be discharged at a velocity higher than a velocity at which the compressed subcooled boil-off gas is introduced; and a supply unit connected to the nozzle unit and configured to supply the liquefied gas stored in the storage unit to the nozzle unit.
The boil-off gas treatment system of claim 2, wherein the phase change unit further includes a cooling unit provided in the nozzle unit and configured to receive the liquefied gas stored in the storage unit and to cool the nozzle unit using cold energy of the liquefied gas.
The boil-off gas treatment system of claim 3, wherein the flow path includes: a converging portion formed such that a flow cross-sectional area of the compressed subcooled boil-off gas gradually decreases along a flow direction of the compressed subcooled boil-off gas; a throat portion provided downstream of the converging portion; and a diverging portion provided downstream of the throat portion and formed such that the flow cross-sectional area of the compressed subcooled boil-off gas gradually increases along the flow direction of the compressed subcooled boil-off gas, wherein the compressed subcooled boil-off gas is accelerated while passing through the throat portion, and the compressed subcooled boil-off gas accelerated through the throat portion expands while passing through the diverging portion.
The boil-off gas treatment system of claim 4, wherein the supply unit includes a transfer pipe connected between the storage unit and the diverging portion and configured to supply the liquefied gas stored in the storage unit into an interior of the diverging portion, wherein the liquefied gas supplied into the interior of the diverging portion through the transfer pipe assists such that an internal temperature of the diverging portion reaches a temperature at which the compressed subcooled boil-off gas is liquefied, or receives cold energy of the compressed subcooled boil-off gas expanded in the diverging portion to be solidified.
The boil-off gas treatment system of claim 4, wherein the cooling unit is disposed closer to the diverging portion than to the converging portion, is installed on an outer surface of the diverging portion, and is provided as a heat-exchange coil configured to receive the liquefied gas stored in the storage unit, wherein cold energy of the liquefied gas supplied to the heat-exchange coil assists a phase change of the compressed subcooled boil-off gas within the diverging portion.
The boil-off gas treatment system of claim 2, wherein the compressed subcooled boil-off gas reliquefied in the nozzle unit and the liquefied gas solidified in the nozzle unit form the liquid/solid-based slush state, and the compressed subcooled boil-off gas in the liquid/solid-based slush state is returned to the storage unit

Description

[Technical Field] The present invention relates to a system for treating boil-off gas. [Background Art] Interest in and demand for liquefied natural gas (LNG), which has a relatively lower carbon emission compared to conventional fossil fuels, have been rapidly increasing worldwide. Such LNG is transported in a gaseous state through onshore or offshore gas pipelines, or is transported to remote consumption locations while being stored in a liquefied state in an LNG carrier. In this case, LNG is obtained by cooling natural gas to a cryogenic temperature, and since the volume thereof is significantly reduced compared to a gaseous state, LNG is highly suitable for long-distance transportation by sea. Meanwhile, an LNG carrier is configured to store LNG and to transport the stored LNG to onshore demand locations. For this purpose, the LNG carrier is provided with a storage tank capable of withstanding the cryogenic temperature of the LNG. In this case, since the liquefaction temperature of LNG is a cryogenic temperature of approximately -163°C at atmospheric pressure, LNG is vaporized even when the temperature of the LNG becomes slightly higher than -163°C at atmospheric pressure. Taking a case of a conventional LNG carrier as an example, although the storage tank of an LNG carrier is thermally insulated, external heat is continuously transferred to LNG. As a result, the LNG is continuously vaporized within the storage tank while being transported by the LNG carrier, thereby generating boil-off gas (BOG) in the storage tank. The boil-off gas generated inside the storage tank increases pressure within the storage tank, and may accelerate the flow of LNG in response to motion of the LNG carrier, thereby causing structural problems in the storage tank. Accordingly, it is necessary to appropriately treat the boil-off gas generated in the storage tank. In addition, since the boil-off gas represents a loss of LNG stored in the storage tank, treatment of the boil-off gas is an important issue in terms of transportation efficiency of LNG. Meanwhile, in order to treat boil-off gas generated in the storage tank of the LNG carrier, in the related art, a method of discharging the boil-off gas from the storage tank to the outside for combustion, a method of discharging the boil-off gas from the storage tank to the outside, re-liquefying the boil-off gas through a re-liquefaction apparatus, and then returning the re-liquefied gas to the storage tank, and a method of using the boil-off gas as a fuel used for a propulsion engine of the vessel have been used alone or in combination. In particular, in the method in which the boil-off gas inside the storage tank is discharged to the outside, re-liquefied through a re-liquefaction apparatus, and then returned to the storage tank, the boil-off gas is expanded using a turbo expander or a pressure-reducing valve in order to re-liquefy the boil-off gas. However, in the case of a turbo expander, although a lower temperature may be reached with a lower compression ratio as compared to a pressure-reducing valve, there is a problem that a large flow rate is required and the system is complex. In addition, in the case of a pressure-reducing valve, although the system is not as complex as that of the turbo expander, there is a problem that a required compression ratio is higher than that of the turbo expander. Furthermore, in order to improve the problems of the conventional turbo expander and pressure-reducing valve, when a multi-stage compressor is used to expand the boil-off gas, more space is required for installing the multi-stage compression, thereby resulting in a problem of poor space utilization. Accordingly, there is need for development of technologies for a boil-off gas treatment system that is not only compactly configured to provide excellent space utilization, but also enables a temperature required for re-liquefaction of boil-off gas to be readily reached even with a lower compression ratio than conventional systems. [Disclosure] [Technical Problem] Embodiments of the present invention have been devised to solve the above-described conventional problems, and to provide a boil-off gas treatment system that is not only compactly configured to provide excellent space utilization, but also enables a temperature required for re-liquefaction of boil-off gas to be readily reached even with a lower compression ratio than conventional systems. [Technical Solution] According to one aspect of the present invention, there is provided a boil-off gas treatment system including: a storage unit in which liquefied gas is stored; a subcooling unit connected to the storage unit and configured to subcool boil-off gas generated from the liquefied gas stored in the storage unit to generate subcooled boil-off gas; a compression unit connected to the subcooling unit and configured to receive the subcooled boil-off gas from the subcooling unit and compress the subcooled boil-off gas to generate comp