KR102183944B1 - System And Method For Treatment Of Boil Off Gas - Google Patents

Abstract

A system and method for treating boil-off gas are disclosed. The boil-off gas treatment system of the present invention is provided in a ship or offshore structure and includes a storage tank for storing low-temperature liquid cargo; A refrigerant circulation unit receiving boil off gas generated from the low-temperature liquid cargo stored in the storage tank and circulating the boil off gas as a refrigerant; A refrigerant supply line for supplying the boil-off gas from the storage tank to the refrigerant circulation unit; And it characterized in that it comprises a shut-off valve provided in the refrigerant supply line.

Description

Boil off gas treatment system and method {System And Method For Treatment Of Boil Off Gas}

The present invention relates to a boil-off gas treatment system and method, and more particularly, to a boil-off gas treatment system and method for circulating boil-off gas generated in a storage tank as a refrigerant by configuring a refrigerant circulation unit.

Recently, the consumption of liquefied gas such as LNG (Liquefied Natural Gas) and LPG (Liquefied Petroleum Gas) is increasing rapidly around the world. The liquefied gas obtained by liquefying the gas at a low temperature has an advantage of increasing storage and transfer efficiency because the volume is very small compared to the gas. In addition, liquefied gas, including liquefied natural gas (hereinafter referred to as "LNG"), can remove or reduce air pollutants during the liquefaction process, so it can be seen as an eco-friendly fuel with little emission of air pollutants during combustion.

For example, liquefied natural gas is a colorless and transparent liquid obtained by liquefying natural gas containing methane as a main component by cooling it to about -162°C, and has a volume of about 1/600 compared to natural gas. Therefore, when transporting natural gas by liquefying it into LNG, it can be transported very efficiently.

However, since the liquefaction temperature of natural gas is a cryogenic temperature of -162 °C, LNG is sensitive to temperature changes and is easily evaporated. For this reason, the LNG storage tank of the LNG carrier is insulated, but since external heat is continuously transferred to the LNG storage tank, the LNG is continuously evaporated naturally in the LNG storage tank during the LNG transportation process and boil-off gas. BOG) occurs. This is the same for other low-temperature liquefied gases such as ethane.

BOG is a kind of loss and is an important problem in transport efficiency. In addition, if the boil-off gas accumulates in the storage tank, the internal pressure of the tank may increase excessively, and in severe cases, there is a risk of damage to the tank. Therefore, various methods for treating BOG generated in storage tanks are studied. Recently, for the treatment of BOG, a method of re-liquefying BOG and returning it to a storage tank, and an energy source of fuel consumers such as ship engines. The method to be used is used.

The present invention is easy to apply to limited in-ship spaces such as ships or offshore structures due to its uncomplicated configuration, and evaporation that can improve transport efficiency and ensure safety by effectively treating evaporated gas generated from a storage tank in which liquid cargo is stored I would like to propose a gas treatment system.

According to an aspect of the present invention, a storage tank provided on a ship or offshore structure to store low temperature liquid cargo;

A refrigerant circulation unit receiving boil off gas generated from the low-temperature liquid cargo stored in the storage tank and circulating the boil off gas as a refrigerant;

A refrigerant supply line for supplying the boil-off gas from the storage tank to the refrigerant circulation unit; And

There is provided a boil-off gas treatment system including a shut-off valve provided in the refrigerant supply line.

A boil-off gas compressor for compressing the boil-off gas generated from the low-temperature liquid cargo, and the boil-off gas provided upstream of the boil-off gas compressor and compressed through at least a portion of the boil-off gas to be introduced into the compressor and the boil-off gas compressor A first heat exchanger for exchanging heat; a second heat exchanger configured to heat-exchange and cool the boiled gas heat-exchanged in the first heat exchanger after compression with the boiled gas circulating in the refrigerant circulation unit; It may further include a first pressure reducing device for reducing the boil-off gas.

Preferably, the refrigerant circulation unit comprises: a refrigerant compressor for compressing the boil-off gas, and a pre-cooler for cooling the boil-off gas compressed by the refrigerant compressor; And a refrigerant pressure reducing device for additional cooling by decompressing the boil-off gas cooled in the pre-cooler,

The boil-off gas cooled through the refrigerant pressure reducing device may be heat-exchanged in the second heat exchanger and circulated to the refrigerant compressor.

Preferably, the separator further comprises a separator provided downstream of the first decompression device to receive the decompressed boil-off gas to separate gas-liquid, and a second decompression device for receiving and decompressing the gas separated from the separator, wherein the separator The separated liquid may be supplied to the storage tank.

The gas reduced by the second decompression device may be supplied from the storage tank to the flow of the boil-off gas to be introduced into the first heat exchanger, or may be supplied to a cold heat supply source through a line supplied to the first heat exchanger.

Preferably, the boil-off gas to be introduced from the pre-cooler to the refrigerant decompression device may be cooled by heat exchange with the boil-off gas depressurized by the refrigerant decompression device and then supplied to the refrigerant decompression device.

Preferably, the boil-off gas compressed by the boil-off gas compressor may be supplied as fuel to gas fuel consumers including a propulsion engine and a power generation engine of the ship or offshore structure.

According to another aspect of the present invention, evaporation characterized in that boil off gas generated from a low-temperature liquid cargo of a storage tank provided in a ship or offshore structure is supplied to a refrigerant circulation unit, and the boil off gas is circulated as a refrigerant. A gas treatment method is provided.

Preferably, the boil-off gas treatment method comprises: 1) compressing the boil-off gas generated in the storage tank with a compressor;

2) cooling the compressed boil-off gas by heat exchange with the boil-off gas to be supplied to the compressor;

3) further cooling the compressed and cooled boil-off gas by exchanging heat with the boil-off gas circulating the refrigerant circulation unit; And

4) It may include the step of decompressing the further cooled boil-off gas to liquefy.

Preferably, the refrigerant circulation unit includes a refrigerant compressor for compressing the boil-off gas, a pre-cooler for cooling the boil-off gas compressed by the refrigerant compressor, and a refrigerant for additional cooling by decompressing the boil-off gas cooled in the pre-cooler. It includes a decompression device, wherein the boil-off gas cooled through the refrigerant decompression device may be heat-exchanged with the boil-off gas compressed and cooled in step 3) and circulated to the refrigerant compressor.

The boil-off gas treatment system of the present invention can re-liquefy the boil-off gas and store it, thereby increasing the transport rate of the liquid cargo and preventing energy waste. In addition, by circulating the boil-off gas as a refrigerant, in particular, by using it as a cold heat source for re-liquefaction of the boil-off gas, it is possible to effectively re-liquefy the boil-off gas without configuring a separate refrigerant cycle, and the economy is high.

In addition, since there is no need to supply a separate refrigerant, it contributes to securing space in the ship. If the refrigerant is insufficient, it can be replenished from the storage tank, so that the refrigerant can be supplied smoothly, and the refrigerant cycle can be effectively operated.

As described above, the present invention simplifies a system configuration for processing boil-off gas on board, and reduces installation and operation costs.

1 schematically shows a boil-off gas treatment system according to an embodiment of the present invention.
2 schematically shows a boil-off gas treatment system according to a first extended embodiment of the present invention.
3 schematically shows a boil-off gas treatment system according to a second extended embodiment of the present invention.

In order to fully understand the present invention, operational advantages of the present invention, and objects achieved by the implementation 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 present invention will be described in detail by describing a preferred embodiment of the present invention with reference to the accompanying drawings. The same reference numerals in each drawing indicate the same member.

Systems for the treatment of boil-off gas to be described later of the present invention include all types of ships and offshore structures equipped with storage tanks capable of storing low-temperature liquid cargo or liquefied gas, that is, LNG carriers, LEG (Liquefied Ethane Gas) carriers, and LNG RVs. It can be applied to marine structures such as ships, LNG FPSO, and LNG FSRU. However, in the embodiments to be described later, for convenience of explanation, LNG, which is a representative low-temperature liquid cargo, will be described as an example.

1 schematically shows a boil-off gas treatment system according to a basic embodiment of the present invention.

As shown in FIG. 1, in the boil-off gas treatment system according to the present embodiment, the boil-off gas NBOG generated and discharged from the storage tank for storing the liquid cargo is transported along the pipe to the boil-off gas compression unit 10. Is compressed.

The storage tank (T) is equipped with sealing and insulating barriers to store liquefied gases such as LNG in a cryogenic state, but it cannot completely block the heat transmitted from the outside, and the evaporation of liquefied gas is continuously performed within the tank. In order to prevent excessive increase in tank pressure due to such boil-off gas, and to maintain an appropriate level of internal pressure, the boil-off gas inside the storage tank is discharged and supplied to the boil-off gas compression unit 10. .

In this embodiment, when the boil-off gas discharged from the storage tank and compressed by the boil-off gas compression unit 10 is referred to as the first stream, the first stream of the compressed boil-off gas is divided into a second stream and a third stream, The stream may be configured to be liquefied and returned to the storage tank T, and the third stream may be configured to be supplied to a gas fuel consumer (not shown) such as a propulsion engine or a power generation engine in the ship. In this case, the boil-off gas compression unit 10 can compress the boil-off gas up to the supply pressure of the fuel consumer, and the second stream can be branched through all or part of the boil-off gas compression unit as necessary. Depending on the fuel requirement of the fuel consumer, all of the compressed boil-off gas may be supplied to the third stream, or the entire amount of the compressed boil-off gas may be returned to the storage tank by supplying the entire amount to the second stream. Gas fuel consumers include high-pressure gas injection engines (for example, ME-GI engines developed by MDT) and low-pressure gas injection engines (for example, Wartsila’s X-DF engine (Generation X-Dual Fuel engine)). ), DF Generator, gas turbine, DFDE, etc.

At this time, a heat exchanger 20 is installed to liquefy the second stream of compressed boil-off gas. In this embodiment, the boil-off gas generated in the storage tank is used as a cold heat supply source for the compressed boil-off gas. While passing through the heat exchanger 20, the compressed boil-off gas whose temperature has risen during the compression process in the boil-off gas compression unit, that is, the second stream, is cooled, and the boil-off gas generated in the storage tank and introduced into the heat exchanger 20 is heated. It is supplied to the boil-off gas compression unit 10.

Since the flow rate of the boil-off gas before being compressed is greater than the flow rate of the second stream, the second stream of the compressed boil-off gas may be supplied with cold heat from the boil-off gas before being compressed to liquefy at least a part. In this way, in the heat exchanger, the low-temperature boil-off gas immediately after discharged from the storage tank and the high-pressure boil-off gas compressed in the boil-off gas compression unit are heat-exchanged to liquefy the high-pressure boil-off gas.

The boil-off gas of the second stream passing through the heat exchanger 20 is further cooled while being depressurized while passing through the expansion means 30 such as an expansion valve or an expander, and is supplied to the separator 40. The liquefied boil-off gas is separated from the gas and liquid components in the separator, and the liquid component, that is, LNG, is returned to the storage tank, and the gas component, that is, the boil-off gas, is discharged from the storage tank, and the heat exchanger 20 and the boil-off gas compression unit ( 10) It may be used as a cold heat supply source for heat exchange of the high-pressure boil-off gas compressed in the boil-off gas compression unit 10 by joining the boil-off gas flow to the boil-off gas flow supplied to the boil-off gas flow, or being supplied to the heat exchanger 20 again. . Of course, it can be sent to a GCU for combustion or sent to a gas consumer (including a gas engine) for consumption.

Another expansion means 50 for further decompressing the gas separated from the separator before joining the boil-off gas flow may be further installed.

Meanwhile, FIG. 2 schematically shows the boil-off gas treatment system of the first extended embodiment in which the above-described basic embodiment is extended.

As shown in FIG. 2, the system of the first extended embodiment includes a refrigerant circulation unit 300a that receives boil off gas generated from a low-temperature liquid cargo stored in a storage tank and circulates the boil-off gas as a refrigerant. It is characterized by the composition.

To this end, it includes a refrigerant supply line (CSLa) that supplies boil-off gas from the storage tank to the refrigerant circulation unit (300a), and a shutoff valve (400a) is provided in the refrigerant supply line to evaporate a sufficient amount to circulate the refrigerant circulation unit. When gas is supplied, the refrigerant supply line CSLa is blocked, so that the refrigerant circulation unit 300a is operated in a closed loop.

As in the above-described basic embodiment, the boil-off gas compressor 100a for compressing boil-off gas generated from the low-temperature liquid cargo in the storage tank T is provided in the first extended embodiment. The boil-off gas generated in the storage tank is introduced into the boil-off gas compressor 100a through the boil-off gas supply line BLa.

The storage tank T of the present embodiments is made of an independent tank type tank in which the load of liquid cargo is not directly applied to the insulation layer, or a membrane type tank in which the load of cargo is directly applied to the insulation layer. I can. In the case of an independent tank type tank, it is also possible to use it as a pressure vessel designed to withstand a pressure of 2 barg or more.

Meanwhile, in the present embodiments, only the line for reliquefaction of the boil-off gas is shown, but the boil-off gas compressed by the boil-off gas compressor is applied to a gas fuel consumer (not shown) including a propulsion engine and a power generation engine of a ship or offshore structure. When the fuel can be supplied and the fuel consumption can consume the entire amount of the boil-off gas, there may be no boil-off gas to be reliquefied. When the consumption of gaseous fuel is small or absent, such as when the ship is at anchor, the entire amount of boil-off gas may be supplied to the reliquefaction line RLA.

The compressed boil-off gas is supplied to the first heat exchanger 200a through the boil-off gas re-liquefaction line RLA, and the first heat exchanger 200a includes the boil-off gas re-liquefaction line Rla and the boil-off gas supply line Bla. Provided throughout, the boil-off gas to be introduced into the boil-off gas compressor (100a) and the boil-off gas compressed through at least a portion of the boil-off gas compressor are heat-exchanged. The boil-off gas whose temperature is increased during the compression process is cooled through heat exchange with the low-temperature boil-off gas that is generated in the storage tank and introduced into the boil-off gas compressor 100a.

A second heat exchanger 500a is provided downstream of the first heat exchanger 200a, and the boil-off gas heat-exchanged in the first heat exchanger after compression is further cooled through heat exchange with the boil-off gas circulating through the refrigerant circulation unit 300a. do.

The refrigerant circulation unit 300a includes a refrigerant compressor 310a for compressing the boil-off gas supplied from the storage tank, a pre-cooler 320a for cooling the boil-off gas compressed by the refrigerant compressor, and the boil-off gas cooled by the pre-cooler. It includes a refrigerant pressure reducing device (330a) for further cooling by reducing the pressure. The refrigerant pressure reducing device may be an expansion valve or expander for cooling by adiabatic expansion of the boil-off gas.

The boil-off gas cooled through the refrigerant decompression device 330a is supplied to the second heat exchanger 500a as a refrigerant along the refrigerant circulation line CCLa, and in the second heat exchanger 500a, the first heat exchanger 200a is The boil-off gas is cooled through heat exchange with the supplied boil-off gas. The boil-off gas of the refrigerant circulation line CCLa that has passed through the second heat exchanger 500a is circulated to the refrigerant compressor 310a, and circulates through the refrigerant circulation line through the above-described compression and cooling process.

Meanwhile, the boil-off gas of the boil-off gas re-liquefaction line Rla cooled in the second heat exchanger 500a is depressurized through the first decompression device 600a. The first pressure reducing device may be, for example, an expansion valve or an expander.

The depressurized boil-off gas is supplied to the separator 700a downstream of the first decompression device 600a to be gas-liquid separated, and the liquid separated by the separator 700a, that is, LNG, is supplied to the storage tank T and stored again.

The gas separated by the separator 700a, that is, the boil-off gas, is further depressurized through the second decompression device 800a, and the boil-off gas is added to the flow of the boil-off gas to be introduced from the storage tank T to the first heat exchanger 200a. It may be used as a cold heat supply source for heat exchange of the high-pressure boil-off gas that is joined to the flow or is supplied to the first heat exchanger 200a and compressed in the boil-off gas compressor 100a. Of course, it can be sent to a GCU for combustion or sent to a gas consumer (including a gas engine) for consumption.

Meanwhile, FIG. 3 schematically shows the boil-off gas treatment system of the second extended embodiment in which the above-described first extended embodiment is further improved.

In the second extended embodiment, the boil-off gas to be introduced from the pre-cooler 320b to the refrigerant decompression device 330b in the refrigerant circulation unit 300b is cooled by heat exchange with the boil-off gas decompressed in the refrigerant decompression device 330b, It is configured to be supplied to the pressure reducing device 330b.

Since the boil-off gas is cooled while being depressurized through the refrigerant decompression device 330b, the boil-off gas downstream of the refrigerant decompression device has a lower temperature than the boil-off gas upstream of the refrigerant decompression device, and this embodiment takes this into account, The boil-off gas is heat-exchanged with the downstream boil-off gas, cooled, and introduced into a decompression device. To this end, as shown in FIG. 3, the boil-off gas upstream of the refrigerant decompression device 330b may be supplied to the second heat exchanger 500b (part A of FIG. 3). If necessary, a separate heat exchange device capable of exchanging heat exchange between the upstream and downstream of the refrigerant decompression device may be additionally configured.

As described above, since the system of the present embodiments can re-liquefy and store the boil-off gas generated in the storage tank liquid cargo, it is possible to increase the transport rate of the liquid cargo. In particular, even when the fuel consumption of the ship's gas consumer is low, in order to prevent an increase in the pressure of the storage tank, the amount of cargo burned and wasted in the GCU can be reduced or eliminated, thereby preventing the waste of energy.

In addition, by circulating the boil-off gas as a refrigerant and using it as a cold heat source for re-liquefying the boil-off gas, it is possible to effectively re-liquefy the boil-off gas without configuring a separate refrigerant cycle. As there is no need to supply a separate refrigerant, the space on board It contributes to securing and is economical. In addition, if the refrigerant is insufficient in the refrigerant cycle, it can be replenished from the storage tank, so that the refrigerant can be replenished smoothly, and the refrigerant cycle can be effectively operated.

In this way, it is possible to re-liquefy the boil-off gas by using the cooling heat of the boil-off gas itself in multiple stages, so that the system configuration for the boil-off gas treatment on board can be simplified, and the cost required for the installation and operation of a device for complex boil-off gas treatment. Can be saved.

As described above, the present invention is not limited to the described embodiments, and it is obvious to those of ordinary skill in the art that various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, it should be said that such modifications or variations belong to the claims of the present invention.

T: storage tank
100a, 100b: boil-off gas compressor
200a, 200b: first heat exchanger
300a, 300b: refrigerant circulation unit
310a, 310b: refrigerant compressor
320a, 320b: pre-cooler
330a, 330b: refrigerant pressure reducing device
400a, 400b: shut-off valve
500a, 500b: second heat exchanger
600a, 600b: first pressure reducing device
700a, 700b: separator
800a, 800b: second pressure reducing device

Claims (10)

A storage tank provided on a ship or offshore structure and storing low temperature liquid cargo;
A boil-off gas compressor for compressing boil off gas generated from the low-temperature liquid cargo;
A refrigerant circulation unit receiving boil-off gas generated from the low-temperature liquid cargo stored in the storage tank and circulating the boil-off gas to a refrigerant;
A refrigerant supply line for supplying the boil-off gas from the storage tank to the refrigerant circulation unit;
A shut-off valve provided in the refrigerant supply line; And
A second heat exchanger configured to cool the compressed boil-off gas by heat exchange with the boil-off gas circulating the refrigerant circulation unit,
The refrigerant circulation unit may include a refrigerant compressor for compressing the boil-off gas; A pre-cooler for cooling the boil-off gas compressed by the refrigerant compressor; And a refrigerant decompression device for further cooling by decompressing the boil-off gas cooled in the pre-cooler, wherein the boil-off gas cooled through the refrigerant decompression device is heat-exchanged in the second heat exchanger. Processing system. The method of claim 1,
A first heat exchanger provided upstream of the boil-off gas compressor and heat-exchanging the boil-off gas to be introduced into the compressor and the boil-off gas compressed through at least a part of the boil-off gas compressor; And
A first decompression device for decompressing the boil-off gas cooled in the second heat exchanger; further comprising,
After compression, the boil-off gas heat-exchanged in the first heat exchanger is cooled by heat exchange with the boil-off gas circulating in the refrigerant circulation unit in the second heat exchanger. The method of claim 2,
The boil-off gas cooled through the refrigerant decompression device in the refrigerant circulation unit is heat-exchanged in the second heat exchanger and circulated to the refrigerant compressor. The method of claim 3,
A separator provided downstream of the first decompression device to receive the depressurized boil-off gas and to separate gas-liquid; And
Further comprising a second pressure reducing device for receiving the gas separated from the separator to reduce pressure,
The boil-off gas treatment system, characterized in that the liquid separated by the separator is supplied to the storage tank. The method of claim 4,
The gas depressurized by the second decompression device is supplied as a flow of the boil-off gas to be introduced from the storage tank to the first heat exchanger, or is supplied to a cold heat supply source through a line supplied to the first heat exchanger. Boil-off gas treatment system. The method according to any one of claims 3 to 5,
The boil-off gas treatment system, characterized in that the boil-off gas to be introduced from the pre-cooler to the refrigerant decompression device is cooled by heat exchange with the boil-off gas depressurized by the refrigerant decompression device and then supplied to the refrigerant decompression device. The method according to any one of claims 2 to 5,
The boil-off gas compressed by the boil-off gas compressor may be supplied as fuel to a gas fuel consumer including a propulsion engine and a power generation engine of the ship or offshore structure. Boil off gas generated from low-temperature liquid cargo in a storage tank provided in a ship or offshore structure is supplied to a refrigerant circulation unit, and the boil-off gas is circulated as a refrigerant,
The refrigerant circulation unit may include a refrigerant compressor for compressing the boil-off gas; A pre-cooler for cooling the boil-off gas compressed by the refrigerant compressor; And a refrigerant pressure reducing device for additional cooling by decompressing the boil-off gas cooled in the pre-cooler,
Compressing the boil-off gas generated in the storage tank with a compressor, and cooling the compressed boil-off gas by heat exchange with the boil-off gas cooled by the refrigerant decompression device of the refrigerant circulation unit. The method of claim 8,
1) compressing the boil-off gas generated in the storage tank with a compressor;
2) cooling the compressed boil-off gas by heat exchange with the boil-off gas to be supplied to the compressor;
3) further cooling the compressed and cooled boil-off gas by exchanging heat with the boil-off gas circulating the refrigerant circulation unit; And
4) decompressing the further cooled boil-off gas to liquefy,
The boil-off gas cooled through the refrigerant decompression device is heat-exchanged with the boil-off gas compressed and cooled in step 3) and circulated to the refrigerant compressor.
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