KR102276361B1 - System and method for reducing gasification rate in lng carrier cargo - Google Patents

Abstract

The present invention relates to a system and method for reducing the vaporization rate of a liquefied natural gas carrier cargo hold using a liquid nitrogen and gas separation unit, comprising: a liquefied natural gas cargo hold provided in a liquefied natural gas carrier; a liquid nitrogen storage tank provided on the upper portion of the liquefied natural gas cargo hold; a liquid nitrogen valve provided in the lower portion of the liquid nitrogen storage tank, which is opened and closed according to the pressure of the liquid natural gas cargo hold and supplies the liquid nitrogen stored in the liquid nitrogen storage tank to the cargo hold; It is provided on the upper part of the LNG cargo hold, and supplies cooling heat to the boil-off gas in the LNG cargo hold and separates the vaporized nitrogen and the boil-off gas that is not reliquefied by the cooling heat of the liquid nitrogen. The BOG generated in the LNG cargo hold is reliquefied by the cooling heat of the liquid nitrogen supplied to the LNG cargo hold, and the BOG and the vaporized nitrogen that are not reliquefied are used in the gas separation unit by using the selective gas permeability of the separation membrane. Disclosed is a system and method for reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier, characterized in that it is separated.

Description

SYSTEM AND METHOD FOR REDUCING GASIFICATION RATE IN LNG CARRIER CARGO

The present invention relates to a system and method for reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier, and more particularly, to a system and method for reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier using a liquid nitrogen and gas separation unit.

In general, natural gas (Natural Gas) is made in a state of liquefied natural gas (Liquefied Natural Gas) liquefied at a cryogenic temperature at a production site and then transported over a long distance to a destination by a liquefied natural gas carrier.

Since the liquefaction temperature of natural gas is a cryogenic temperature of -163 ℃ at normal pressure, liquefied natural gas is evaporated even if its temperature is slightly higher than -163 ℃ at normal pressure. Although the liquefied natural gas cargo hold of a liquefied natural gas carrier is insulated, external heat is continuously transferred to the liquefied natural gas, so the liquefied natural gas is liquefied during transportation of the liquefied natural gas by the liquefied natural gas carrier. Boil-off gas is generated in the liquefied natural gas cargo hold as it is continuously vaporized in the natural gas cargo hold.

If boil-off gas is generated in the liquefied natural gas cargo hold in this way, the pressure in the liquefied natural gas storage tank rises, making it dangerous.

Conventionally, in order to maintain the pressure of the LNG cargo hold in a safe state, when the pressure in the LNG cargo hold rises above the set pressure, the BOG is discharged to the outside of the cargo hold, and the discharged BOG is propelled by the LNG carrier. Although used as a fuel, the steam turbine propulsion method, which is driven by steam generated by burning boil-off gas generated from the liquefied natural gas cargo hold in a boiler, has a problem with low propulsion efficiency.

In addition, the dual fuel diesel electric propulsion system, which compresses the boil-off gas generated from the LNG cargo hold and uses it as fuel for a diesel engine, has higher efficiency than the steam turbine propulsion method, but Since the electric propulsion device is complicated, there are many difficulties in the maintenance of the equipment. In addition, since this method has to supply BOG as fuel, a gas compression method having a higher installation and operating cost than liquid compression is inevitably applied. And, this method of using boil-off gas as a fuel for propulsion does not reach the efficiency of a two-stroke low-speed diesel engine used in general ships in any case.

As described above, liquefied natural gas generates boil-off gas, which is a natural gas that is vaporized due to a natural vaporization phenomenon that inevitably occurs during transportation, and enormous costs are consumed to re-liquefy it. Accordingly, efforts to reduce the cost are continuing, and in order to reduce the cost, it is necessary to improve the insulation performance of the liquefied natural gas carrier cargo hold and the re-liquefaction facility.

Conventionally, as a method for reducing the vaporization rate of the liquefied natural gas, a technique for adjusting the material and application size of an insulation box or insulation panel applied to an insulation system of a LNG cargo hold, or re-liquefying the vaporized boil-off gas Techniques including a reliquefaction device have been developed, but there is still a problem in that a huge cost is required.

Therefore, in the present invention, in order to solve the above problems, liquid nitrogen is supplied to the liquefied natural gas cargo hold to re-liquefy the boil-off gas generated in the liquefied natural gas cargo hold, thereby reducing the vaporization rate, and re-liquefying natural gas and vaporization. It is a technical solution to provide a system for reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier using liquefied nitrogen and a gas separation unit, so that the nitrogen is separated by the gas separation unit.

In addition, the present invention is another solution to provide a method of reducing the vaporization rate of the liquefied natural gas carrier cargo hold using the liquid nitrogen and gas separation unit.

According to one aspect of the present invention for solving the above problems, a liquefied natural gas cargo hold provided on a liquefied natural gas carrier;

a liquid nitrogen storage tank provided on the upper portion of the liquefied natural gas cargo hold;

a liquid nitrogen valve provided in the lower portion of the liquid nitrogen storage tank, which is opened and closed according to the pressure of the liquid natural gas cargo hold and supplies the liquid nitrogen stored in the liquid nitrogen storage tank to the cargo hold;

It is provided on the upper part of the LNG cargo hold, and supplies cooling heat to the boil-off gas in the LNG cargo hold and separates the vaporized nitrogen and the boil-off gas that is not reliquefied by the cooling heat of the liquid nitrogen.

The BOG generated in the LNG cargo hold is reliquefied by the cooling heat of the liquid nitrogen supplied to the LNG cargo hold, and the BOG and the vaporized nitrogen that are not reliquefied are used in the gas separation unit by using the selective gas permeability of the separation membrane. A system for reducing vaporization rate of a cargo hold of a liquefied natural gas carrier is provided, characterized in that it is separated.

Preferably, the liquid nitrogen valve opens the liquid nitrogen storage tank when the pressure in the liquid natural gas cargo hold exceeds a set pressure, and supplies the stored liquid nitrogen to the liquid natural gas cargo hold of the liquid nitrogen storage tank. Opening and closing can be adjusted.

Preferably, the gas separation unit includes: a natural gas separation membrane for selectively permeating and discharging only the boil-off gas by receiving non-reliquefied BOG and vaporized nitrogen from the liquefied natural gas cargo hold; and an outlet for discharging the vaporized nitrogen that does not permeate through the separation membrane.

Preferably, the natural gas separation membrane may be formed of PTMSP (poly(1-trimethylsily-1-propyne)).

Preferably, it may further include a natural gas collection unit for collecting boil-off gas separated and discharged through the gas separation unit.

Preferably, the boil-off gas collected in the natural gas collecting unit may be supplied as fuel in the carrier, or may be liquefied by the cooling heat of the liquid nitrogen through one side of the liquid nitrogen storage tank and re-stored in the cargo hold.

According to another aspect of the present invention, the first step of supplying the liquid nitrogen from the liquid nitrogen storage tank in which liquid nitrogen is stored to the liquid natural gas cargo hold;

a second step of re-liquefying the boil-off gas generated in the LNG cargo hold by cooling and heat of the supplied liquefied nitrogen in the LNG cargo hold; and

Supplying the nitrogen vaporized in the second step and the boil-off gas that is not reliquefied to a gas separation unit having selective gas permeability, comprising a third step of separating the vaporized nitrogen and the boil-off gas,

A method of reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier using liquid nitrogen is provided.

Preferably, the gas separation unit, a gas separation membrane selectively permeating natural gas; and an outlet for discharging the residual gas that has not passed through the gas separation membrane, wherein the vaporized nitrogen and non-reliquefied BOG supplied to the gas separation unit pass through the gas separation membrane, and the BOG is selectively transmitted, the The vaporized nitrogen that has not passed through the separation membrane may be discharged through the outlet, so that the vaporized nitrogen and the boil-off gas may be separated.

Preferably, the gas separation membrane may be formed of PTMSP (poly(1-trimethylsily-1-propyne)).

Preferably, the method may further include a fourth step of collecting the boil-off gas separated in the third step, supplying it as fuel in the ship, or liquefying it with the cooling heat of the liquid nitrogen and re-storing it in the cargo hold.

According to the present invention described above, by supplying liquefied nitrogen into the liquefied natural gas cargo hold, re-liquefaction can be made in the liquefied natural gas cargo hold, and the non-reliquefied boil-off gas or vaporized nitrogen can be separated in the gas separation unit, nitrogen It has the effect of reducing the loss of natural gas that is discarded together with the gas.

In addition, according to the present invention, the gas separation unit uses a separation membrane that selectively permeates natural gas to separate natural gas through a dissolution and diffusion process, so that it does not accompany a phase change, so energy consumption is low, installation area is small, and maintenance has an easy effect.

1 schematically shows a liquefied natural gas cargo hold and a liquefied nitrogen storage tank according to the present invention.
2 schematically shows a gas separation unit using a selective gas permeable membrane according to the present invention.

Hereinafter, the configuration and operation of the preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. In addition, the following examples may be modified in various other forms, and the scope of the present invention is not limited to the following examples.

1 schematically shows a liquefied natural gas carrier cargo hold and a liquefied nitrogen storage tank according to an embodiment of the present invention.

As shown in Figure 1, the vaporization rate reduction system of the cargo hold of the LNG carrier of this embodiment is liquefied by supplying liquid nitrogen from the liquid nitrogen storage tank 20 at the upper portion of the LNG cargo hold 10 into the LNG cargo hold. Boiled gas and vaporized nitrogen that are reliquefied in the natural gas cargo hold and are not reliquefied may be separated from natural gas in the gas separation unit 30 , and the separated natural gas may be collected to prevent loss of natural gas.

Therefore, the present invention is a system for reducing the vaporization rate of the LNG carrier cargo hold as an aspect, the LNG cargo hold 10; a liquid nitrogen storage tank 20 provided on the upper portion of the liquefied natural gas cargo hold; a liquid nitrogen valve 21 provided at a lower portion of the liquid nitrogen storage tank and controlling the supply of the stored liquid nitrogen to the cargo hold by controlling the opening and closing of the liquid nitrogen storage tank; It is provided on the upper portion of the liquefied natural gas cargo hold and includes a gas separation unit 30 for separating the non-liquefied boil-off gas and vaporized nitrogen discharged from the cargo hold.

In addition, the boil-off gas that has not been reliquefied due to the cooling and heat of liquid nitrogen supplied to the liquefied natural gas cargo hold by the liquid nitrogen valve and the vaporized nitrogen while reliquefying the boil-off gas are discharged to the gas separation unit 30 for controlling , a natural gas valve 11 may be provided in the upper portion of the cargo hold.

Through such a system, in this embodiment, the vaporization rate can be reduced by re-liquefying and storing the boil-off gas generated in the liquefied natural gas cargo hold by the cooling heat of liquefied nitrogen supplied to the liquefied natural gas cargo hold.

In this embodiment, the liquid nitrogen storage tank 20 is provided in the upper portion of the liquefied natural gas cargo hold 10, the liquid nitrogen valve 21 is provided in the lower portion of the liquid nitrogen storage tank 20, liquefied By controlling the opening and closing of the liquid nitrogen storage tank 20 by the nitrogen valve 21, liquid nitrogen is supplied to the LNG cargo hold.

Since the liquefied natural gas has a boiling point of -163 ℃ at normal pressure, even if the temperature of the liquefied natural gas is slightly higher than -163 ℃, the evaporated natural gas, that is, boil-off gas, is generated, and liquefied natural gas is easily evaporated. The pressure in the gas cargo hold 10 is increased, and when the pressure inside the LNG cargo hold 10 exceeds a predetermined pressure due to the boil-off gas, the liquid nitrogen valve 21 causes the liquefaction. The nitrogen storage tank 20 is opened, and liquid nitrogen is supplied to the liquefied natural gas cargo hold 10 .

On the other hand, since liquid nitrogen supplied from the storage tank 20 to the cargo hold 10 has a boiling point of -196° C. at normal pressure, the liquid nitrogen supplied into the LNG cargo hold 10 is evaporated through heat exchange due to the difference in boiling point. The gas is cooled and re-liquefied into liquefied natural gas, and in this process, liquid nitrogen is evaporated by depriving it of cooling heat. Therefore, the vaporization rate of the liquefied natural gas is reduced by re-liquefying the liquefied natural gas by vaporization of the liquefied nitrogen.

In addition, in this embodiment, a natural gas valve 11 is provided on the upper portion of the liquefied natural gas cargo hold 10, and nitrogen vaporized in the cargo hold 10 and the boil-off gas that has not been reliquefied are separated from the gas separation unit. (30) is discharged.

In addition, the gas separator 30 separates the boil-off gas and vaporized nitrogen supplied by the natural gas valve 11 .

Specifically, since the gas separator uses a separation membrane having selective gas permeability, BOG and vaporized nitrogen can be separated without a phase change through a dissolution and diffusion process, thereby reducing energy consumption required for gas separation.

FIG. 2 schematically shows only the gas separation unit according to the present embodiment, and may include a natural gas separation membrane 31 and an outlet 32 .

At this time, by using a selective permeable membrane as the natural gas separation membrane 31, only the BOG is selectively transmitted and discharged from the non-reliquefied BOG and vaporized nitrogen supplied from the liquefied natural gas cargo hold, and the natural gas Vaporized nitrogen that has not passed through the separation membrane 31 is discharged through the outlet 32 to separate boil-off gas and vaporized nitrogen.

In addition, PTMSP (poly(1-trimethylsily-1-propyne)) has a selective gas permeability to natural gas that is about three times higher than that of nitrogen gas, so the separation membrane formed of PTMSP is selectively permeable to separate boil-off gas and vaporized nitrogen. It can be applied as a membrane.

In this way, the natural gas that has passed through the BOG by using the selective permeation membrane can move back to the cargo hold. At this time, by installing the natural gas valve 11 provided on the upper part of the liquefied natural gas cargo hold 10 through one side of the liquefied nitrogen storage tank 20 positioned on the upper part of the cargo hold, it is evaporated through heat exchange with liquid nitrogen. The gas may be liquefied and stored in the cargo hold 10 .

In addition, the system of this embodiment further includes a natural gas collection unit (not shown) for collecting natural gas separated and discharged through the gas separation unit 30, by recovering the natural gas permeated from the selective permeation membrane. It can be used by supplying it as fuel for propulsion or power generation. The natural gas collected in the natural gas collecting unit may be re-liquefied through one side of the liquid nitrogen storage tank 20 and re-stored in the cargo hold 10 .

In addition, the vaporized nitrogen gas that has not passed through the selective permeable membrane may be discharged into the atmosphere through the outlet 32 .

As described above, in this embodiment system, a first step of supplying the stored liquid nitrogen from the liquid nitrogen storage tank in which liquid nitrogen is stored to the liquid natural gas cargo hold; a second step of re-liquefying the boil-off gas by supplying cooling heat to the boil-off gas generated in the liquid natural gas cargo hold while the liquid nitrogen supplied from the storage tank is vaporized in the LNG cargo hold; and supplying the nitrogen vaporized in the second step and the boil-off gas that has not been re-liquefied to the gas separation unit, and passing through a third step of separating the vaporized nitrogen and the boil-off gas, thereby re-using the boil-off gas by using the cooling heat of liquid nitrogen. It is possible to reduce the vaporization rate of the cargo hold of a liquefied natural gas carrier by liquefying it.

In particular, when the pressure in the cargo hold exceeds a preset pressure due to the boil-off gas generated in the LNG cargo hold 10 as described above, the liquid nitrogen valve 21 is opened and liquid nitrogen is discharged from the liquid nitrogen storage tank 20 . is supplied to the cargo hold, and nitrogen vaporized while reliquefying BOG in the cargo hold and BOG that has not been reliquefied are supplied to the gas separation unit provided on the upper part of the cargo hold, and the vaporized nitrogen and BOG are separated from each other and discharged. .

Specifically, the boil-off gas contained in vaporized nitrogen is separated through a gas separation membrane made of PTMSP (poly(1-trimethylsily-1-propyne)) with excellent selective permeability to natural gas, and the remaining gas cannot pass through the separation membrane. Nitrogen is separated from each other by venting through the outlet. The loss of natural gas can be prevented by collecting the boil-off gas separated in this way and supplying it to various fuels in the ship, or re-liquefying it and storing it in the cargo hold.

The present invention is not limited to the described embodiments, and it is apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit and scope of the present invention. Accordingly, it should be said that such modifications or variations are included in the claims of the present invention.

10: LNG cargo hold
11: natural gas valve
20: liquid nitrogen storage tank
21: liquid nitrogen valve
30: gas separation unit
31: natural gas separation membrane
32: outlet

Claims (10)

Liquefied natural gas cargo hold provided on a liquefied natural gas carrier;
a liquid nitrogen storage tank provided on the upper portion of the liquefied natural gas cargo hold;
a liquid nitrogen valve provided in the lower portion of the liquid nitrogen storage tank, which is opened and closed according to the pressure of the liquid natural gas cargo hold and supplies the liquid nitrogen stored in the liquid nitrogen storage tank to the cargo hold;
a gas separation unit provided in the upper portion of the liquefied natural gas cargo hold, supplying cooling heat to the boil-off gas in the LNG cargo hold, and separating vaporized nitrogen and the boil-off gas that is not reliquefied by the cooling heat of the liquid nitrogen; and
A natural gas valve provided at the upper portion of the liquefied natural gas cargo hold to control the discharge of boil-off gas and vaporized nitrogen that are not reliquefied due to the cooling and heat of liquid nitrogen to the gas separation unit: including:
The BOG generated in the LNG cargo hold is reliquefied by the cooling heat of the liquid nitrogen supplied to the LNG cargo hold, and the BOG and the vaporized nitrogen that are not reliquefied are used in the gas separation unit by using the selective gas permeability of the membrane The vaporization rate reduction system of the cargo hold of a liquefied natural gas carrier, characterized in that it is separated. The method of claim 1,
The liquid nitrogen valve opens the liquid nitrogen storage tank when the pressure in the LNG cargo hold exceeds a set pressure, and controls the opening and closing of the liquid nitrogen storage tank to supply the stored liquid nitrogen to the LNG cargo hold A system for reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier using liquid nitrogen, characterized in that. The method of claim 1,
The gas separation unit,
a natural gas separation membrane for selectively permeating and discharging only the boil-off gas by receiving non-reliquefied BOG and vaporized nitrogen from the LNG cargo hold; and
A system for reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier using liquid nitrogen, characterized in that it includes; an outlet for discharging the vaporized nitrogen that does not pass through the separation membrane. 4. The method of claim 3,
The natural gas separation membrane is PTMSP (poly(1-trimethylsily-1-propyne)), characterized in that formed of, the vaporization rate reduction system of the liquefied natural gas carrier cargo hold using liquid nitrogen. The method of claim 1,
The vaporization rate reduction system of a cargo hold of a liquefied natural gas carrier using liquid nitrogen, characterized in that it further comprises a natural gas collection unit for collecting boil-off gas separated and discharged through the gas separation unit. 6. The method of claim 5,
The boil-off gas collected in the natural gas collecting unit is supplied as fuel in the carrier, or is liquefied by cooling and heat of the liquid nitrogen through one side of the liquid nitrogen storage tank, characterized in that it is re-stored in the cargo hold, liquid nitrogen A system for reducing the vaporization rate of the cargo hold of a liquefied natural gas carrier using A first step of supplying the liquid nitrogen from the liquid nitrogen storage tank in which liquid nitrogen is stored to the liquid natural gas cargo hold;
a second step of re-liquefying the boil-off gas generated in the LNG cargo hold by the cooling heat of the supplied liquefied nitrogen in the LNG cargo hold; and
By controlling the natural gas valve provided on the upper part of the liquefied natural gas cargo hold to supply the nitrogen vaporized and non-reliquefied BOG in the second step to a gas separation unit having selective gas permeability, to separate the vaporized nitrogen and BOG comprising the third step,
A method of reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier using liquid nitrogen. 8. The method of claim 7,
The gas separation unit, a gas separation membrane selectively permeating natural gas; and an outlet for discharging the residual gas that has not passed through the gas separation membrane,
The vaporized nitrogen and non-reliquefied BOG supplied to the gas separation unit are selectively transmitted through the gas separation membrane, and the vaporized nitrogen that has not passed through the separation membrane is discharged through the outlet, the vaporization A method of reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier using liquid nitrogen, characterized in that nitrogen and boil-off gas are separated. 9. The method of claim 8,
The gas separation membrane is PTMSP (poly(1-trimethylsily-1-propyne)), characterized in that formed of, a method of reducing the vaporization rate of a cargo hold of a liquefied natural gas carrier using liquid nitrogen. 8. The method of claim 7,
Liquefaction using liquid nitrogen, characterized in that it further comprises a fourth step of collecting the boil-off gas separated in the third step, supplying it as fuel in the ship, or liquefying it with the cooling heat of the liquid nitrogen and re-storing it in the cargo hold A method for reducing the vaporization rate of the cargo hold of a natural gas carrier.

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