KR20220087621A - Boil-Off Gas Reliquefaction System And Method For Ship - Google Patents

Abstract

A system and method for reliquefying BOG of a vessel are disclosed. The BOG reliquefaction system of a ship of the present invention includes: a compressor for compressing BOG generated from an onboard storage tank; a heat exchanger for cooling the boil-off gas compressed in the compressor by heat exchange with a refrigerant; a refrigerant circulation unit in which the refrigerant supplied to the heat exchanger circulates; a boil-off gas supply line connected from the storage tank to the compressor through the heat exchanger; a heating line branched from the heat exchanger upstream of the boil-off gas supply line to heat all or part of the boil-off gas and supply it to the front end of the heat exchanger of the boil-off gas supply line; and a re-liquefaction line for cooling and re-liquefying the boil-off gas compressed in the compressor in the heat exchanger to return to the storage tank.

Description

Boil-Off Gas Reliquefaction System And Method For Ship

The present invention relates to a system and method for re-liquefying boil-off gas (BOG) generated from liquefied gas stored in a storage tank of a ship by cooling and re-liquefying it, and minimizing thermal stress of a heat exchanger. will be.

Natural gas, which has methane as a main component, is receiving attention as an eco-friendly fuel because there is almost no emission of environmental pollutants during combustion. Liquefied Natural Gas (LNG) is obtained by cooling natural gas to about -163°C under normal pressure and liquefying it. very suitable Therefore, natural gas is mainly stored and transported in the state of liquefied natural gas, which is easy to store and transport.

Since the liquefaction point of natural gas is a cryogenic temperature of about -163°C at atmospheric pressure, it is common that the LNG storage tank is insulated to maintain the liquid state of the LNG. However, although the LNG storage tank is insulated, it has a limitation in blocking external heat, and since the external heat is continuously transmitted to the LNG storage tank, during the LNG transportation process, the LNG is continuously and naturally in the LNG storage tank. It is vaporized and boil-off gas (BOG) is generated.

If boil-off gas is continuously generated in the LNG storage tank, it becomes a factor to increase the internal pressure of the LNG storage tank. If the internal pressure of the storage tank exceeds the set safety pressure, it may cause an emergency such as tank rupture, so the boil-off gas must be discharged to the outside of the storage tank using a safety valve. However, since BOG is a kind of LNG loss and is an important problem in transport efficiency and fuel efficiency of LNG, various methods for treating BOG generated in the storage tank are being used.

Recently, a method of using BOG in fuel demanders such as engines of ships, a method of re-liquefying BOG to a storage tank, or a method using a combination of these two methods, etc. have been developed and applied.

As a method for re-liquefying BOG, a method of re-liquefying BOG by heat-exchanging BOG with a refrigerant by having a refrigeration cycle using a separate refrigerant, a method of re-liquefying BOG itself as a refrigerant without a separate refrigerant, etc. There is this.

A PRS (Partial Re-liquefaction System) that reliquefies compressed BOG by heat exchange with uncompressed BOG and adiabatic expansion to re-liquefy the compressed BOG by using BOG itself as a refrigerant without a separate refrigerant; and Improved technology has also been developed and applied to ships.

As a system using a separate refrigeration cycle, a reliquefaction process using a nitrogen refrigerant may be mentioned.

The nitrogen refrigerant has relatively low efficiency compared to a cycle using a mixed refrigerant, but has high safety because the refrigerant is inert, and has the advantage of being easier to apply to a ship because there is no phase change of the refrigerant.

In this way, the BOG cooled by the cooling heat of a separate refrigerant or BOG itself is introduced into the separator, and the reliquefied gas separated from gas-liquid is recovered to the storage tank.

However, in a BOG reliquefaction system in which a heat exchanger is provided, significant thermal stress may be applied to the heat exchanger during start-up of the reliquefaction system or when the temperature of BOG is changed due to a change in cargo tank condition, which may damage the device. can cause

In order to solve this problem, the present invention intends to propose a BOG reliquefaction system capable of reducing thermal stress applied to a heat exchanger.

According to one aspect of the present invention for solving the above problems, a compressor for compressing boil-off gas generated from the storage tank on board;

a heat exchanger for cooling the boil-off gas compressed in the compressor by heat exchange with a refrigerant;

a refrigerant circulation unit in which the refrigerant supplied to the heat exchanger circulates;

a boil-off gas supply line connected from the storage tank to the compressor through the heat exchanger;

a heating line branched from the heat exchanger upstream of the boil-off gas supply line to heat all or part of the boil-off gas and supply it to the front end of the heat exchanger of the boil-off gas supply line; and

A reliquefaction line for cooling and reliquefying the BOG compressed in the compressor in the heat exchanger and returning it to the storage tank is provided.

Preferably, a heater provided in the heating line to heat the boil-off gas; a first valve provided between a branch point and a merging point of the heating line in the boil-off gas supply line; a second valve provided upstream of the heater in the heating line; and a temperature control unit provided at the rear end of the junction of the heating line in the boil-off gas supply line to sense the temperature of the boil-off gas introduced into the heat exchanger to control the first and second valves.

Preferably, the refrigerant circulation unit comprises: an expander in which the refrigerant to be supplied to the heat exchanger is expanded and cooled; a refrigerant compressor for cooling the boil-off gas in the heat exchanger and compressing the refrigerant discharged; and a refrigerant circulation line connected from the refrigerant compressor to the expander through the heat exchanger, and from the expander to the refrigerant compressor through the heat exchanger to circulate the refrigerant.

Preferably, a pressure reducing device provided in the reliquefaction line to depressurize the boil-off gas cooled in the heat exchanger; and a separator for gas-liquid separation of the boil-off gas decompressed in the decompression device after cooling through the heat exchanger.

Preferably, a liquid level sensor for detecting the liquid level inside the separator; and a liquid level control valve provided downstream of the separator to transfer the liquefied gas separated from the separator to the storage tank.

Preferably, the refrigerant compressor receives the expansion energy of the refrigerant from the expander to compress the refrigerant, and the refrigerant compressor and the expander may be provided as a compander.

According to another aspect of the present invention, the boil-off gas generated from the onboard storage tank is compressed in a compressor, and the compressed gas compressed in the compressor is cooled and reliquefied by heat exchange with a refrigerant in a heat exchanger,

BOG from the storage tank is supplied to the compressor through the heat exchanger,

All or part of the boil-off gas supplied from the storage tank to the compressor is branched and heated upstream of the heat exchanger to join the front end of the heat exchanger,

There is provided a method for reliquefying BOG of a ship, characterized in that it is possible to control the temperature of BOG introduced from the storage tank to the heat exchanger.

Preferably, all or part of the BOG supplied from the storage tank to the compressor is transferred upstream of the heat exchanger when the reliquefaction system is started-up or when the temperature of the BOG is changed due to a change in the condition of the storage tank. By branching and heating in the heat exchanger to join the front end, it is possible to increase the temperature of the boil-off gas introduced into the heat exchanger to reduce the thermal stress of the heat exchanger.

Preferably, a refrigerant circulating along a refrigerant circulation line is supplied to the heat exchanger, and the refrigerant in the refrigerant circulation line is compressed by a refrigerant compressor and then supplied to an expander through the heat exchanger, and after expansion and cooling in the expander, the heat exchange Four flows of boil-off gas to be supplied to the compressor from the storage tank in the heat exchanger, boil-off gas to be compressed and re-liquefied in the compressor, refrigerant compressed in the refrigerant compressor, and refrigerant expanded and cooled in the expander in the heat exchanger This can be heat exchanged.

Preferably, the boil-off gas cooled in the heat exchanger is further cooled under reduced pressure and then gas-liquid separated and returned to the storage tank, and the refrigerant compressor receives the expansion energy of the refrigerant from the expander and compresses the refrigerant, the refrigerant The compressor and expander may be provided as a compander.

In the present invention, the reliquefaction rate can be increased by more effectively cooling the BOG to be reliquefied by using the cooling heat of the BOG itself and the cooling heat of the refrigerant cycle.

In particular, in the present invention, by allowing the temperature of BOG introduced to the heat exchanger to be adjusted, when the BOG temperature changes due to a change in the condition of the storage tank or start-up of the reliquefaction system, the temperature of the BOG is rapidly changed to the heat exchanger. Prevents excessive thermal stress from occurring, prevents device damage, and enables stable operation of the reliquefaction system.

1 schematically shows a BOG reliquefaction system of a ship according to an embodiment of the present invention.

In order to fully understand the operational advantages of the present invention and the objects achieved by the practice of the present invention, reference should be made to the accompanying drawings illustrating preferred embodiments of the present invention and the contents described in the accompanying drawings.

Hereinafter, the configuration and operation of the preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Here, it should be noted that in adding reference signs to the elements of each drawing, the same elements are indicated with the same reference numerals as much as possible even though they are indicated on different drawings.

In an embodiment of the present invention to be described later, the vessel may be any type of vessel in which a storage tank for storing liquefied gas is provided. Typical examples include ships with self-propelled capabilities such as LNG carriers, liquid hydrogen carriers, and LNG RVs (Regasification Vessels), as well as LNG Floating Production Storage Offloading (FPSO) and LNG FSRU (Floating Storage Regasification Unit). It may also include offshore structures that do not have the capability but are floating in the sea.

In addition, this embodiment can be transported by liquefying the gas at a low temperature, and can be applied to a re-liquefaction cycle of all types of liquefied gas in which boil-off gas is generated in a stored state. Such liquefied gas is, for example, liquefied petrochemicals such as LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), Liquefied Ethylene Gas, Liquefied Propylene Gas, etc. It may be gas. However, in the embodiments to be described later, an example in which LNG, which is a representative liquefied gas, is applied will be described.

1 schematically shows a BOG reliquefaction system of a ship according to an embodiment of the present invention.

1, the BOG reliquefaction system of this embodiment is a system for reliquefying BOG generated from a storage tank T in which liquefied gas is stored in the ship and returning it to the storage tank, and supplies BOG Compressor 100 for receiving and compressing, BOG supply line (GL) for supplying BOG generated from the storage tank to the compressor, Reliquefaction line (GL) connected from the compressor to the storage tank to reliquefy BOG and return to the storage tank ( RL).

The BOG supply line GL is connected from the storage tank to the compressor 100 through the heat exchanger 200, so that the uncompressed BOG generated in the storage tank is supplied to the compressor after supplying cooling heat to the heat exchanger and compressed.

The compressor 100 compresses the boil-off gas, for example, it may be compressed by the fuel supply pressure of the ship's main engine. For example, when the DF engine is provided, the compression can be 5.5 barg, when the X-DF engine is provided, it can be compressed at 15 barg, and when the ME-GI engine is provided, the compression can be 300 barg. The compressed BOG may also be supplied as a fuel of the ship's main engine (not shown), and may re-liquefy BOG that is not supplied as fuel.

In accordance with ship regulations, the compressor that supplies fuel to the engine must be designed for redundancy in preparation for emergency situations, so one compressor is shown in the drawing, but the compressor may be configured including a main compressor and a redundant compressor, It may be provided as a multi-stage compressor in which a plurality of compressors and an intercooler are provided.

BOG compressed in the compressor 100 is introduced into the heat exchanger 200 along the reliquefaction line RL and cooled through heat exchange.

In the reliquefaction line (RL), a heat exchanger 200 for cooling the boil-off gas compressed in the compressor, a pressure reducing device 400 for decompressing the boil-off gas cooled by heat exchange, and the boil-off gas cooled through the heat exchanger and the decompression device. A separator 500 for supplying liquefied gas to the storage tank by separating gas-liquid is provided.

In the heat exchanger 200 , the boil-off gas is cooled through heat exchange with the refrigerant circulating in the refrigerant circulation unit 300 .

The refrigerant circulation unit 300 includes a refrigerant circulation line CL through which the refrigerant circulates, and the refrigerant circulation line cools the expander 310 in which the refrigerant to be supplied to the heat exchanger is expanded and cooled, and the boil-off gas discharged from the heat exchanger is cooled. and a refrigerant compressor 320 for compressing the discharged refrigerant is provided. The refrigerant compressor and the expander may be shaft-connected to provide a compander that uses the expansion energy of the refrigerant to compress the refrigerant, thereby reducing power required to drive the refrigerant cycle.

As the refrigerant supplied to the heat exchanger while circulating the refrigerant circulation line CL, for example, nitrogen (N ₂ ) may be used.

The refrigerant compressed in the refrigerant compressor 320 is cooled in the heat exchanger 200, then expanded and cooled in the expander 310, is supplied as the refrigerant of the heat exchanger 200 again, and circulates through the refrigerant circulation line CL. Accordingly, in the heat exchanger 200, four flows of the boil-off gas compressed in the compressor, the uncompressed boil-off gas to be introduced into the compressor, the refrigerant expanded and cooled in the expander, and the refrigerant compressed in the refrigerant compressor are heat exchanged.

BOG cooled while passing through the heat exchanger may be further cooled by adiabatic expansion or isentropic expansion through decompression in the decompression device 400 and then gas-liquid separated in the separator.

A liquid level control valve (LV) is provided downstream of the separator 500 in the reliquefaction line (RL), and a liquid level sensor (LC) for detecting the liquid level inside the separator is provided. By controlling the valve (LV), the reliquefied gas separated from the separator is transferred to the storage tank (T).

However, when operating the BOG reliquefaction system as described above, when the temperature of BOG is changed due to a start-up of the reliquefaction system or a change in the condition of the storage tank, thermal stress of the heat exchanger may be induced.

In particular, a heat exchanger such as a brazed aluminum heat exchanger that may be installed in a nitrogen refrigeration cycle of the refrigerant circulation unit is vulnerable to thermal stress. The temperature of boil-off gas generated in the storage tank and introduced to the heat exchanger is usually around -100°C, and depending on the storage tank condition, boil-off gas of -130°C or lower may be generated in the storage tank. When such a cryogenic boil-off gas is introduced into the heat exchanger at room temperature at the initial start-up by operating the stopped reliquefaction system or the heat exchanger before sufficient cool-down is achieved, significant thermal stress is generated. This can cause damage to the heat exchanger equipment.

In this embodiment, in order to solve this problem, a heating line (HL) capable of adjusting the temperature of the boil-off gas introduced to the heat exchanger is configured in the boil-off gas supply line (GL) at the front end of the heat exchanger 200, to minimize stress.

As shown in FIG. 1 , a heating line HL that heats all or part of the boil-off gas upstream of the heat exchanger 200 of the boil-off gas supply line GL and supplies it to the front end of the heat exchanger of the boil-off gas supply line is branched, , a heater 150 for heating the boil-off gas is provided in the heating line. As a heat source of the heater, for example, glycol water, steam, seawater, or fresh water may be used.

In the boil-off gas supply line (GL), a first valve (V1) is provided between the branching point and the merging point of the heating line, and a second valve (V2) is provided upstream of the heater in the heating line (HL), directly from the storage tank. The amount of boil-off gas introduced into the heat exchanger and the boil-off gas heated through the heater and introduced into the heat exchanger is controlled. In the boil-off gas supply line (GL), the temperature control unit (TC) is provided at the rear end of the junction of the heating line, and the opening and closing degree of the first and second valves (V1, V2) while sensing the temperature of the boil-off gas introduced into the heat exchanger can control

For example, when the heat exchanger at room temperature is not sufficiently cooled during start-up of the reliquefaction system, the BOG supplied from the storage tank is sensed through the temperature controller TC while sensing the BOG temperature at the front end of the heat exchanger. All or a part of is heated in the heater 150 by branching to the heating line HL, and is supplied to the heat exchanger 200 together with the boil-off gas flow that does not pass through the heater. While sensing the temperature of BOG continuously introduced into the heat exchanger through the temperature control unit TC, the amount of BOG that does not go through the heater by gradually lowering the opening degree of the second valve V2 and increasing the opening degree of the first valve V1 The temperature of the boil-off gas (cold BOG) introduced to the heat exchanger is gradually decreased by increasing

Even during normal operation of the reliquefaction system, if there is a risk that thermal stress may occur in the heat exchanger due to a change in the temperature of the boil-off gas due to a change in the condition of the storage tank, all or part of the boil-off gas supplied from the storage tank to the compressor is transferred upstream of the heat exchanger. It is possible to reduce the thermal stress of the heat exchanger by branching from and heating it through a heating line and controlling the temperature of the boil-off gas introduced into the heat exchanger. As such, it is possible to prevent thermal fatigue and device damage of the heat exchanger through the system according to the present embodiment, and to stably operate the reliquefaction system.

The present invention is not limited to the above embodiments, and it is apparent to those skilled in the art that various modifications or variations can be implemented without departing from the technical gist of the present invention. did it

T: storage tank
GL: BOG supply line
RL: reliquefaction line
CL: Refrigerant circulation line
HL: Heating line
V1, V2: first and second valves
TC: temperature control unit
100: compressor
150: heater
200: heat exchanger
300: refrigerant circulation unit
310: inflator
320: refrigerant compressor
400: pressure reducing device
500: separator

Claims (10)

Compressor for compressing boil-off gas generated from the onboard storage tank;
a heat exchanger for cooling the boil-off gas compressed in the compressor by heat exchange with a refrigerant;
a refrigerant circulation unit in which the refrigerant supplied to the heat exchanger circulates;
a boil-off gas supply line connected from the storage tank to the compressor through the heat exchanger;
a heating line branched from the heat exchanger upstream of the boil-off gas supply line to heat all or part of the boil-off gas and supply it to the front end of the heat exchanger of the boil-off gas supply line; and
BOG reliquefaction system of a ship comprising: a reliquefaction line for cooling and reliquefying BOG compressed in the compressor in the heat exchanger and returning it to the storage tank. The method of claim 1,
a heater provided in the heating line to heat the boil-off gas;
a first valve provided between a branch point and a merging point of the heating line in the boil-off gas supply line;
a second valve provided upstream of the heater in the heating line; and
BOG reliquefaction of a vessel further comprising: a temperature control unit provided at the rear end of the junction of the heating line in the BOG supply line and sensing the temperature of BOG introduced to the heat exchanger to control the first and second valves system. The method of claim 2, wherein the refrigerant circulation unit,
an expander in which the refrigerant to be supplied to the heat exchanger is expanded and cooled;
a refrigerant compressor for cooling the boil-off gas in the heat exchanger and compressing the refrigerant discharged; and
and a refrigerant circulation line connected from the refrigerant compressor to the expander through the heat exchanger, and from the expander to the refrigerant compressor through the heat exchanger to circulate the refrigerant. 4. The method of claim 3,
a pressure reducing device provided in the reliquefaction line to depressurize the boil-off gas cooled in the heat exchanger; and
A separator for gas-liquid separation of the boil-off gas decompressed in the decompression device after cooling through the heat exchanger. 5. The method of claim 4,
a liquid level sensor detecting a liquid level inside the separator; and
BOG reliquefaction system of a vessel further comprising: a liquid level control valve provided downstream of the separator to transfer the liquefied gas separated from the separator to the storage tank. 6. The method according to any one of claims 1 to 5,
The refrigerant compressor receives the expansion energy of the refrigerant from the expander and compresses the refrigerant, and the refrigerant compressor and the expander are provided as a compander. The boil-off gas generated from the onboard storage tank is compressed by a compressor, and the compressed gas compressed in the compressor is cooled and reliquefied by heat exchange with a refrigerant in a heat exchanger,
BOG from the storage tank is supplied to the compressor through the heat exchanger,
All or a part of the boil-off gas supplied from the storage tank to the compressor is branched and heated upstream of the heat exchanger to join the front end of the heat exchanger,
BOG reliquefaction method of a ship, characterized in that it is possible to adjust the temperature of BOG introduced from the storage tank to the heat exchanger. 8. The method of claim 7,
At the time of start-up of the reliquefaction system or when the temperature of the boil-off gas changes due to a change in the condition of the storage tank,
All or part of the BOG supplied from the storage tank to the compressor is branched upstream of the heat exchanger and heated to join the front end of the heat exchanger, thereby increasing the temperature of BOG introduced into the heat exchanger to increase the thermal stress of the heat exchanger. BOG reliquefaction method of a ship, characterized in that reducing the. 9. The method of claim 8,
A refrigerant circulating along a refrigerant circulation line is supplied to the heat exchanger, the refrigerant in the refrigerant circulation line is compressed by a refrigerant compressor and then supplied to an expander through the heat exchanger, and is introduced as a refrigerant to the heat exchanger after expansion and cooling in the expander Became,
In the heat exchanger, four flows of the boil-off gas to be supplied from the storage tank to the compressor, the boil-off gas to be compressed and reliquefied by the compressor, the refrigerant compressed in the refrigerant compressor, and the refrigerant expanded and cooled by the expander are heat-exchanged A method of re-liquefying boil-off gas in ships. 10. The method according to claim 8 or 9,
The boil-off gas cooled in the heat exchanger is further cooled under reduced pressure and then gas-liquid separated and returned to the storage tank,
The refrigerant compressor receives the expansion energy of the refrigerant from the expander and compresses the refrigerant, and the refrigerant compressor and the expander are provided as a compander.

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