KR20210072848A - Boil-Off Gas Reliquefaction Method and System for Vessels - Google Patents

Abstract

The present invention relates to a boil-off gas (BOG) re-liquefaction method for a ship, which re-liquefies BOG generated by naturally vaporizing liquefied gas, and a system using the same. The BOG re-liquefaction method for a ship comprises: a compression step of compressing BOG, discharged from a liquefied gas storage tank, to a pressure required by a high-pressure engine; a cooling step of cooling the high-pressure BOG compressed in the compression step, by exchanging heat between the high-pressure BOG and BOG discharged from the liquefied gas storage tank and transferred to the compression step; a first decompression step of first depressurizing the BOG cooled in the cooling step; and a second decompression step of further decompressing the BOG, decompressed in the first decompression step, to a final target pressure. The first decompression step includes a pressure adjustment step of adjusting the degree of decompression of the BOG in the first decompression step so that the pressure of the BOG transferred from the first decompression step to the second decompression step remains constant. Therefore, a re-liquefaction process can be performed smoothly.

Description

Boil-Off Gas Reliquefaction Method and System for Vessels}

The present invention relates to a method and system for re-liquefying boil-off gas (BOG) generated by natural vaporization of liquefied gas.

In recent years, consumption of liquefied gas such as liquefied natural gas (LNG) is rapidly increasing worldwide. The liquefied gas obtained by liquefying the gas at a low temperature has the advantage of increasing the storage and transport efficiency because the volume is very small compared to the gas. In addition, since liquefied gas including liquefied natural gas can remove or reduce air pollutants during the liquefaction process, it can be viewed as an eco-friendly fuel that emits less air pollutants during combustion.

Liquefied natural gas is a colorless and transparent liquid obtained by cooling and liquefying natural gas containing methane to about -162°C, and has a volume of about 1/600 compared to natural gas. Accordingly, when the natural gas is liquefied and transported, it can be transported very efficiently.

However, since the liquefaction temperature of natural gas is a cryogenic temperature of -162 ℃ atmospheric pressure, liquefied natural gas is sensitive to temperature changes and evaporates easily. For this reason, the storage tank that stores the liquefied natural gas is insulated, but since external heat is continuously transferred to the storage tank, the liquefied natural gas is continuously naturally vaporized in the storage tank during the process of transporting the liquefied natural gas and boils off gas (Boil). -Off Gas, BOG) occurs. This is also true for other low-temperature liquefied gases such as ethane.

BOG is a type of loss and is an important problem in transport efficiency. In addition, when boil-off gas is accumulated 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 the storage tank are studied. Recently, for the treatment of BOG, a method of re-liquefying BOG and returning it to the storage tank, and turning BOG into fuel such as engines of ships, etc. A method of using it as an energy source for a consumer is being used.

As a method for re-liquefying BOG, a method of re-liquefying BOG by heat exchange 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. In particular, a system employing the latter method is called a partial reliquefaction system (PRS).

On the other hand, as engines that can use natural gas as fuel among engines generally used in ships, there are gas fuel engines such as DF engines and ME-GI engines.

DF engines (DFDE, DFDG) are composed of 4 strokes, and are based on the Otto Cycle, which injects natural gas with a relatively low pressure of 6.5 bar into the combustion air inlet and compresses it as the piston rises. works with

The ME-GI engine is composed of two strokes, and operates based on a diesel cycle in which high-pressure natural gas near 300 bar is directly injected into the combustion chamber near top dead center of the piston. Recently, there is a growing interest in ME-GI engines with better fuel efficiency and propulsion efficiency.

2 shows a conventional BOG reliquefaction system. Referring to FIG. 2 , the boil-off gas generated by natural vaporization of LNG in the storage tank T is compressed using the compressor C and supplied as fuel for the engines MEGI and DFDE. BOG that is not supplied to the engine, that is, compressed BOG exceeding the fuel demand of the engine, is branched to the re-liquefaction device to be re-liquefied and recovered.

The compressed BOG to be reliquefied is cooled by heat exchange with BOG before compression transferred from the storage tank (T) to the compressor (C) in the heat exchanger (H), and the temperature is further increased as the pressure is reduced by the decompression device (D). lowers BOG whose pressure and temperature have been lowered by the decompression device (D) is separated from gas-liquid in the gas-liquid separator (S), and the reliquefied BOG in a liquid state is recovered to the storage tank (T), and non-liquefied BOG in a gaseous state is joined to the boil-off gas flow before compression supplied to the heat exchanger (H).

The compressor (C) is a multi-stage compressor, and may compress the boil-off gas to the pressure required by the high pressure engine (MEGI), that is, about 300 barg over several stages. The high-pressure BOG to be reliquefied among the 300 barg high-pressure BOG compressed in the compressor (C) is transferred to the heat exchanger (H) and cooled by heat exchange. The high-pressure boil-off gas of about 300 barg cooled in the heat exchanger (H) is liquefied by adiabatic expansion to about 3 barg in the pressure reducing device (D).

Here, the pressure reducing device D, as shown in FIG. 2, is provided with a first pressure reducing device D1 and a second pressure reducing device D2 in series, and 300 barg of high pressure boil-off gas is produced in two steps. A configuration that reduces the pressure to barg is applied. This is because it is more efficient to depressurize the high-pressure BOG over several steps than to depressurize the BOG over a single step using a single decompression device (D).

If only one decompression device (D) is provided and the pressure is reduced from a high pressure of 300 barg to 3 barg at a time, the pressure drop is too large and a large amount of flash gas may be generated, so that the reliquefaction efficiency is reduced, and the decompression device Because the load in (D) is large, the device cost increases.

When the high-pressure BOG is reduced from 300 barg to 3 barg in two steps using two decompression devices (D), first, the first decompression device (D1) upstream reduces the BOG of 300 barg to 70 barg. It is provided as a pressure reducing device for depressurizing, and the downstream second pressure reducing device D2 is provided as a pressure reducing device for decompressing the boil-off gas of 70 barg to 3 barg. That is, when the two pressure reducing devices have the same load or the pressure reducing device is a valve, the opening degree of each valve is set to be the same.

The first pressure reducing device D1 and the second pressure reducing device D2 are means for liquefying the boil-off gas by lowering the pressure and temperature by depressurizing the gaseous boil-off gas, and the first pressure reducing device D1 and the second pressure reducing device ( D2) is a means for depressurizing the gaseous fluid.

However, the inventors of the present invention found that the primary reduced pressure boil-off gas supplied from the first pressure reducing device D1 to the second pressure reducing device D2 is in a completely liquefied state, or a gas-liquid two-phase mixed state. A problem was found that made it difficult to control the reliquefaction flow rate.

The temperature of the high-pressure BOG cooled in the heat exchanger (H) varies depending on the pressure (or flow rate) and temperature of the BOG supplied from the storage tank (T) as a refrigerant of the heat exchanger (H). That is, when the temperature of the BOG supplied as a refrigerant to the heat exchanger (H) is lowered, the temperature of the high-pressure BOG cooled in the heat exchanger (H) is lowered. Therefore, when the temperature of the high-pressure BOG supplied from the heat exchanger H to the first pressure reducing device D1 is lower than the set value, the BOG supplied from the first pressure reducing device D1 to the second pressure reducing device D2 is reduced. The state may be a liquid or gas-liquid mixed state. If the boil-off gas supplied to the second decompression device D2 is in a liquid or gas-liquid mixed state, it is difficult to control the flow rate of the boil-off gas, and re-liquefaction of the boil-off gas is not performed smoothly.

Accordingly, the present invention is to solve the above-described problem, and the high-pressure BOG cooled in the heat exchanger is reduced to a set pressure in two steps, and the BOG decompressed in the first decompression device is in a liquid or gas-liquid mixed state. It is an object of the present invention to provide a method and system for reliquefying BOG for ships, so that the reliquefaction process is smoothly carried out.

According to one aspect of the present invention for achieving the above object, the compression step of compressing the boil-off gas discharged from the liquefied gas storage tank to a pressure required by the high-pressure engine; a cooling step of cooling the high-pressure BOG compressed in the compression step by exchanging heat with BOG discharged from the liquefied gas storage tank and transferred to the compression step; a first decompression step of first depressurizing the boil-off gas cooled in the cooling step; and a second decompression step of further depressurizing the boil-off gas decompressed in the first decompression step to a final target pressure, wherein the first decompression step includes, in the first decompression step, evaporation transferred from the first decompression step to the second decompression step In order to keep the pressure of the gas constant, a pressure control step of adjusting the degree of decompression of the boil-off gas in the first decompression step is provided.

Preferably, in the first decompression step and the second decompression step, the pressure of the boil-off gas is respectively performed by a pressure reducing valve, and the temperature of the boil-off gas is lowered while the pressure is reduced by the pressure reducing valve, and the pressure adjusting step includes: The first pressure reducing step may be performed by controlling the opening rate of the first pressure reducing valve for decompressing the boil-off gas, and the opening rate of the second pressure reducing valve for decompressing the boil-off gas in the second pressure reducing step may be fixed.

Preferably, the pressure adjusting step comprises: a pressure measuring step of measuring the pressure of the boil-off gas transferred from the first decompression step to the second decompression step; and an opening rate adjusting step of adjusting the opening rate of the first pressure reducing valve for decompressing the boil-off gas in the first decompression step according to the pressure measurement value measured in the pressure measuring step.

Preferably, in the step of adjusting the opening rate, the basic opening rate corresponding to the intermediate target pressure targeted in the first decompression step and the pressure measurement value in the pressure measuring step are increased or decreased to the intermediate target pressure. By comparing the required variable opening rate, the opening rate of the first pressure reducing valve may be set as the larger value of the opening rate.

Preferably, the BOG supplied from the first decompression step to the second decompression step may be in a gaseous state or a supercritical state, and the BOG decompressed in the second decompression step may be in a liquid state or a gas-liquid mixed state.

Preferably, the method may further include a recovery step of supplying the reliquefied BOG in a liquid state decompressed in the second decompression step to the liquefied gas storage tank.

According to another aspect of the present invention for achieving the above object, a compression step of compressing the boil-off gas discharged from the liquefied gas storage tank to the pressure required by the high-pressure engine; a cooling step of cooling the high-pressure BOG compressed in the compression step by exchanging heat with BOG discharged from the liquefied gas storage tank and transferred to the compression step; a first decompression step of first depressurizing the boil-off gas cooled in the cooling step; and a second decompression step of further depressurizing the boil-off gas decompressed in the first decompression step to a final target pressure, wherein the first decompression step includes, in the first decompression step, evaporation transferred from the first decompression step to the second decompression step In order to prevent the state of the gas from becoming a liquid state or a gas-liquid mixed state, a pressure control step of adjusting the degree of decompression of the boil-off gas in the first decompression step is provided.

Preferably, the pressure adjusting step comprises: a pressure measuring step of measuring the pressure of the boil-off gas transferred from the first decompression step to the second decompression step; and adjusting an opening rate of a pressure reducing valve for decompressing boil-off gas in the first decompression step according to the pressure measurement value of the pressure measuring step. The opening rate may be a fixed value.

In addition, according to another aspect of the present invention for achieving the above object, the compressor for compressing the boil-off gas discharged from the liquefied gas storage tank to the pressure required by the high-pressure engine; a heat exchanger configured to heat the high-pressure BOG compressed in the compressor with BOG discharged from the liquefied gas storage tank and transferred to the compressor to cool the high-pressure BOG; a first pressure reducing device for first depressurizing the boil-off gas cooled in the heat exchanger; a second pressure reducing device for further reducing the boil-off gas decompressed in the first pressure reducing device to a final target pressure; a pressure measuring unit for measuring the pressure between the first pressure reducing device and the second pressure reducing device; and a control unit for controlling the degree of decompression of the boil-off gas by the first pressure reducing device so that the pressure measurement value of the pressure measurement unit is maintained above a set value.

Preferably, the first pressure reducing device is a Joule-Thomson valve for expanding the boil-off gas by an isenthalpy process, and the control unit includes an opening rate of the first pressure reducing device corresponding to an intermediate target pressure and measuring the pressure and a high selector configured to set the opening rate of the first pressure reducing device to a larger value by comparing the opening rate of the first pressure reducing device required to increase or decrease the negative pressure measurement value to the intermediate target pressure.

Preferably, the second pressure reducing device may be a Joule-Thomson valve that expands the boil-off gas by an isenthalpy process, and the opening rate of the second pressure reducing device may be a fixed value that does not change.

The method and system for reliquefying BOG for ships according to the present invention reduce the high-pressure BOG cooled in a heat exchanger to a set pressure in two steps, and the BOG decompressed in the first decompression device is in a liquid or gas-liquid mixed state. so that the reliquefaction process is carried out smoothly.

1 is a schematic diagram illustrating a BOG reliquefaction system according to an embodiment of the present invention.
2 is a schematic diagram illustrating a conventional BOG reliquefaction system.

In order to fully understand the operational advantages of the present invention and the objects achieved by the embodiments 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, in adding reference signs to the elements of each drawing, it should be noted that only the same elements are indicated by the same reference signs as possible even if they are indicated on different 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.

Liquefied gas in an embodiment of the present invention to be described later may be applied to various liquefied gases, for example, LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), liquefied gas It may be a liquefied petrochemical gas such as ethylene gas (Liquefied Ethylene Gas) or liquefied propylene gas (Liquefied Propylene Gas). Alternatively, liquid gas such as liquefied carbon dioxide, liquefied hydrogen or liquefied ammonia may be used. However, in the embodiments to be described later, an example in which LNG, which is a representative liquefied gas, is applied will be described.

In addition, although the BOG treatment system and method according to an embodiment of the present invention, which will be described later, is described as an example applied to a ship, it may be applied on land.

In addition, in the embodiment to be described later, the ship is described by taking the case of a liquefied natural gas carrier (LNG Carrier) that transports liquefied natural gas as cargo, but the present invention is an LNG FSRU (Floating LNG FSRU) equipped with a storage tank for storing liquefied natural gas. Liquefied gas storage tanks such as Storage Regasification Unit), LNG FPSO (Floating Production Storage Offloading), and LNG RV (Regasification Vessel) are provided, and an engine supplied with liquefied gas as fuel is applied. It is applicable to all ships equipped with a reliquefaction device for reliquefying.

In addition, in an embodiment of the present invention to be described later, the engine may include any two or more of a high-pressure engine, a medium-pressure engine, and a low-pressure engine. The high-pressure engine is described as an engine using gas fuel of about 100 bar to 400 bar, or about 150 bar or more, preferably about 300 bar, for example, an ME-GI engine. Also, the medium pressure engine may be an engine using a gaseous fuel of about 10 bar to 20 bar, preferably about 16 bar, for example an X-DF engine, and the low pressure engine may be about 5 bar to 10 bar, preferably about 16 bar. It may be an engine using gas fuel of about 6.5 bar, for example, a DF engine, a DFDG engine, or a DFGE engine.

In an embodiment of the present invention to be described later, a high-pressure engine and a low-pressure engine, that is, a ME-GI engine and a DFDE will be described as an example.

In addition, the BOG reliquefaction system according to the present embodiment may be applied to a partial reliquefaction system (PRS) or a full reliquefaction system (FRS) that improves the partial reliquefaction system to increase the reliquefaction amount. .

Hereinafter, a vessel BOG reliquefaction method and a vessel BOG reliquefaction system according to an embodiment of the present invention will be described with reference to FIG. 1 .

A vessel BOG reliquefaction system according to an embodiment of the present invention includes an LNG storage tank 100 for storing LNG; a multi-stage compressor 300 for compressing the boil-off gas transferred from the LNG storage tank 100 along the boil-off gas line (BL) to the pressure required by the high-pressure engine (MEGI); and a re-liquefaction device for re-liquefying the remaining BOG that is not supplied to the engine among the high-pressure BOG compressed by the multi-stage compressor 300 and recovering it to the LNG storage tank 100 .

The reliquefaction apparatus of this embodiment includes a heat exchanger 200 for cooling the remaining BOG that is not supplied to the engine among the high-pressure BOG compressed by the multi-stage compressor 300; a first pressure reducing device 610 for decompressing the boil-off gas cooled in the heat exchanger 200 to an intermediate target pressure; and a second decompression device 620 for further decompressing the boil-off gas of the intermediate pressure decompressed by the first decompression device 620 to a final decompression target pressure to liquefy it.

In addition, the pressure measuring unit 630 for measuring the pressure of the boil-off gas discharged after being decompressed from the first decompression device 610 of the present embodiment; And by adjusting the degree of BOG decompression of the first pressure reducing device 610 according to the pressure measurement value of the pressure measuring unit 630 , the pressure between the first pressure reducing device 610 and the second pressure reducing device 620 is constant. It further includes; a control unit 640 for maintaining.

In addition, the BOG reliquefaction system for ships according to the present embodiment may further include a gas-liquid separator 700 for gas-liquid separation of BOG liquefied in the second pressure reducing device 620 .

In the LNG storage tank 100 of this embodiment, LNG may be stored at about 1.1 bar, -162°C. BOG (BOil-Off Gas) generated by natural vaporization of LNG in the LNG storage tank 100 is discharged from the LNG storage tank 100 and transferred along the boil-off gas line BL, and the multi-stage compressor 300 After being compressed in the fuel line (FL1, FL2), it is supplied to the engine (MEGI, DFDE) to be used as fuel for the engine, or is reliquefied while being transported along the reliquefaction line (RL) to the LNG storage tank (100). is recovered again

The boil-off gas line BL connects the LNG storage tank 100 and the heat exchanger 200 and the heat exchanger 200 and the multi-stage compressor 300 , and the boil-off gas discharged from the LNG storage tank 100 is boil-off gas. While flowing along the line BL, cooling heat is recovered while cooling the high-pressure BOG in the heat exchanger 200 , and then transferred to the multi-stage compressor 300 .

The multi-stage compressor 300 may compress the boil-off gas to the pressure required by the high pressure engine (MEGI) over several stages including a plurality of cylinders and a plurality of inter-coolers.

In this embodiment, as shown in FIG. 1, the multi-stage compressor 300 includes five cylinders and five intercoolers, and a five-stage compressor capable of compressing BOG over at least one stage to five stages is applied. An example will be described.

Among the plurality of cylinders of the multi-stage compressor 300 , at least one of the cylinders at the rear end may include an oil-lubricating type cylinder. In this embodiment, three cylinders of the rear stage are oil-lubricated cylinders, and the two cylinders of the front stage are oil-free lubrication type cylinders as an example. Therefore, although oil is not mixed in the low-pressure BOG compressed through two stages using only the two cylinders in the front stage, the high pressure compressed through the five stages using both the two cylinders in the front and three cylinders in the rear BOG may contain oil.

The pressure of the low-pressure BOG compressed in two stages using the two cylinders of the oil-free lubrication method at the front end of the multi-stage compressor 300 of this embodiment is the low-pressure engine, that is, the pressure of gas fuel required by the DFDE in this embodiment. , that is, about 5 bar to 10 bar, or about 6.5 bar.

According to this embodiment, as shown in Fig. 1, some of the cylinders of the multi-stage compressor 300, that is, among the low-pressure BOG compressed in two stages using two cylinders of the oil-free lubrication method in this embodiment, The low-pressure boil-off gas corresponding to the amount of gas fuel required by the low-pressure engine may be branched from the boil-off gas line BL to the second fuel line FL2 and supplied as gas fuel of the low pressure engine. The second fuel line FL2 is branched from the boil-off gas line BL at the downstream of the second cylinder of the multi-stage compressor 300 and is connected to the low-pressure engine.

In addition, the pressure of the high-pressure BOG compressed over five steps using both the two oil-free lubrication cylinders at the front end and the three oil-lubricated lubrication cylinders at the rear end of the multi-stage compressor 300 of this embodiment is the main component of the boil-off gas. It may be above the supercritical pressure of phosphorus methane. And the pressure of the boil-off gas compressed through five steps in the multi-stage compressor 300 is about 150 bar or more, or about 100 bar to 400 bar, which is the pressure of gas fuel required in the high-pressure engine, that is, the ME-GI engine in this embodiment. , or about 300 bar.

In addition, as shown in FIG. 1 , the high-pressure BOG as much as the amount of gas fuel required by the high-pressure engine among the high-pressure BOG compressed over five steps using all five cylinders of the multi-stage compressor 300 is the first fuel. It is supplied as gaseous fuel of the high-pressure engine through line FL1. The first fuel line FL1 connects the multi-stage compressor 300 and the high-pressure engine MEGI.

Among the high-pressure BOG compressed through 5 stages using all 5 cylinders of the multi-stage compressor 300, the remaining high-pressure BOG, except for the amount of gas fuel required by the high-pressure engine (MEGI), is branched to the reliquefaction line (RL) to be re-liquefied. After liquefaction, it can be recovered to the LNG storage tank 100 .

The reliquefaction line RL may be branched from the first fuel line FL1 , and the high-pressure boil-off gas is transferred to the heat exchanger 200 , the first pressure reducing device 610 , the second pressure reducing device 620 , and the gas-liquid separator 700 . ) is connected to be recovered to the LNG storage tank 100 .

According to this embodiment, at least one installed downstream of the multi-stage compressor 300 of the first fuel line FL1 or upstream of the heat exchanger 200 of the re-liquefaction line RL, and compressed in the multi-stage compressor 300, oil A first oil filter 510 for removing oil from the mixed high-pressure boil-off gas; may further include.

Here, the high-pressure BOG transferred from the multi-stage compressor 300 to the heat exchanger 200 along the reliquefaction line RL may be transferred to the heat exchanger 200 after the oil is filtered by the first oil filter 510 . .

The first oil filter 510 may be a means suitable for filtering liquid and/or vapor-state fluids from supercritical or gaseous fluids. That is, the first oil filter 510 may filter oil in a liquid and/or vapor state from the high-pressure boil-off gas in the supercritical state.

The high-pressure BOG from which oil has been removed from the first oil filter 510 is supplied to the heat exchanger 200 along the reliquefaction line RL. By supplying the high-pressure boil-off gas from which the oil has been removed to the heat exchanger 200, the clogging of the heat exchanger 200 by oil can be prevented.

The heat exchanger 200 of this embodiment may be a heat exchanger having a micro-channel type flow path. For example, the heat exchanger 200 may be a PCHE (Printed Circuit Heat Exchanger), which is a heat exchanger suitable for exchanging cryogenic BOG and high pressure BOG.

As described above, in a heat exchanger having a microchannel flow path, clogging by oil occurs more frequently, and the clogging by oil has a fatal effect on reliquefaction performance. Therefore, it is preferable to supply the boil-off gas from which the oil is removed to the heat exchanger 200 after oil is removed from the high-pressure boil-off gas upstream of the heat exchanger 200 as in the present embodiment.

In the heat exchanger 200, the high-pressure BOG cooled by heat exchange with BOG transferred from the LNG storage tank 100 to the multi-stage compressor 300 along the BOG line BL is converted into a reliquefaction line RL. ) is supplied to the first pressure reducing device 610 along the line.

The first pressure reducing device 610 of the present embodiment may be an expansion valve for reducing the high-pressure BOG to an intermediate pressure by an isenthalpy process, for example, a Joule-Thomson valve, or reducing the high-pressure BOG by an isentropic process. It may be an inflator. In this embodiment, the first pressure reducing device 610 is a Joule-Thomson valve will be described as an example.

In the first pressure reducing device 610, the high-pressure BOG is reduced in temperature as the pressure is reduced.

BOG whose pressure and temperature have been lowered in the first pressure reducing device 610 is transferred to the second pressure reducing device 620 along the reliquefaction line RL, and the BOG is pressured to the target pressure in the second pressure reducing device 620 . this lowers

The second pressure reducing device 620 of the present embodiment may be an expansion valve for reducing the high-pressure BOG to an intermediate pressure by an isenthalpy process, for example, a Joule-Thomson valve, or a high-pressure BOG by an isentropic process. It may be an inflator that depressurizes it. In this embodiment, the second pressure reducing device 620 will be described by taking as an example a Joule-Thompson valve.

In this embodiment, the pressure of the boil-off gas reduced in the first pressure reducing device 610, that is, the intermediate target pressure, may be about 70 barg, and the pressure of the boil-off gas reduced in the second pressure reducing device 620, that is, the final target pressure. The pressure may be about 3 barg.

In addition, according to the present embodiment, the first pressure reducing device 610 is the pressure of the boil-off gas transferred from the first pressure reducing device 610 to the second pressure reducing device 620 measured using the pressure measuring unit 630 . The opening rate may be adjusted according to the measured value.

Conditions such as the pressure, flow rate and temperature of the boil-off gas transferred along the boil-off gas line BL, used as a refrigerant in the heat exchanger 200 , and/or heat exchange along the reliquefaction line RL in the multi-stage compressor 300 . The temperature and pressure of the boil-off gas transferred from the heat exchanger 200 to the first pressure reducing device 610 varies according to conditions such as the flow rate of the boil-off gas transferred to the unit 200 .

That is, if the opening rate of the first pressure reducing device 610 is a fixed value or is controlled simultaneously with the second pressure reducing device 620 only according to the amount of liquidation, the pressure downstream of the first pressure reducing device 610 is adjusted to the intermediate target pressure. There may be cases in which it does not reach or exceeds the intermediate target pressure excessively.

In particular, when the pressure downstream of the first pressure reducing device 610 is lower than the intermediate target pressure, the boil-off gas tends to be in a liquid or gas-liquid two-phase mixed state.

According to this embodiment, the opening rate of the second pressure reducing device 620 , that is, the valve position is set to be fixed, and the opening rate of the first pressure reducing device 610 is controlled according to the pressure measurement value of the pressure measuring unit 630 . The control unit 640 for maintaining the pressure between the first pressure reducing device 610 and the second pressure reducing device 620 equal to or greater than the intermediate target pressure; may further include.

The control unit 640 of this embodiment includes a high selector (HS) of the first pressure reducing device 610 , and the set pressure of the first pressure reducing device 610 , that is, the intermediate pressure and the pressure measuring unit 630 . By comparing the measured pressure values, it is possible to select and control a larger value among the corresponding opening rates.

For example, according to the present embodiment, the control unit 640 if the pressure measurement value of the pressure measurement unit 640 is lower than the set value (intermediate target pressure), that is, about 70 barg, the intermediate pressure of this embodiment, 1 By increasing the opening rate of the pressure reducing device 610 , the pressure drop at the front and rear ends of the first pressure reducing device 610 is reduced, and thus the pressure downstream of the first pressure reducing device 610 is increased to increase the pressure of the first pressure reducing device 610 and the second pressure reducing device 610 . The pressure between the decompression devices 620 may be kept constant.

On the other hand, even if the opening rate of the first pressure reducing device 610 is adjusted in this way, the opening rate of the second pressure reducing device 620 is fixed, so that the total amount of liquefaction is not affected.

In addition, in the present embodiment, it has been described as an example that the opening rate of the first pressure reducing device 610 is feedback-controlled by the pressure measurement value of the pressure measuring unit 630 , but the heat exchanger 200 and the first pressure reducing device 610 . ), the opening rate of the first pressure reducing device 610 may be controlled according to the temperature measurement value.

In addition, the BOG reliquefaction system for ships according to the present embodiment is installed at one or more positions of the downstream of the first decompression device 610 and the downstream of the second decompression device 620 , and the pressure reducing device 610, It may further include; a second oil filter 520 for removing the oil remaining in the boil-off gas decompressed in 620).

Even when the first oil filter 510 is installed upstream of the heat exchanger 200, some oil, particularly in a vapor state, is not filtered well and remains in the boil-off gas and undergoes a re-liquefaction process together with the boil-off gas. Although there are differences depending on the composition, the oil's condensation point is higher than that of boil-off gas (methane), so the oil solidifies through the reliquefaction process.

Therefore, as in the present embodiment, a second oil filter 520 is additionally installed downstream of the decompression devices 610 and 620 , so that the reliquefied BOG is reliquefied and evaporated before being recovered to the LNG storage tank 100 . By additionally filtering the oil from the gas, it is possible to solve the problem that the oil contaminates the LNG storage tank 100 .

The second oil filter 520 of this embodiment may be a cryogenic filter suitable for filtering out solids from liquid fluids.

In addition, the second oil filter 520 is installed between the gas-liquid separator 700 of the second pressure reducing device 520 , and the reliquefied BOG from which oil has been filtered in the second oil filter 520 is transferred to the gas-liquid separator 700 . can be transported

In the gas-liquid separator 700 , non-condensed BOG in a gaseous state such as flash gas generated while the BOG is decompressed from the reliquefied BOG and BOG naturally vaporized in the gas-liquid separator 700 and reliquefaction evaporation in a liquid state Gas is separated into gas-liquid.

The non-condensed BOG in the gaseous state separated in the gas-liquid separator 700 is discharged from the gas-liquid separator 700 along the gas recovery line GL, and through the BOG line BL upstream of the heat exchanger 200, the LNG storage tank It may be joined to the boil-off gas flow supplied as a refrigerant of the heat exchanger 200 from 100 .

In the related art, the valve positions of the first pressure reducing device 610 and the second winding device 620 are fixed without adjusting the opening degree of the first pressure reducing device 610, and the first pressure reducing device 610 and the second pressure reducing device 610 according to the amount of liquefaction are fixed. Unlike the simultaneous operation of two pressure reducing devices 620, according to the present invention, by adjusting the opening rate of the first pressure reducing device 610 to maintain the pressure downstream of the first pressure reducing device 610 above a certain pressure, The second pressure reducing device 620 prevents the boil-off gas of liquid or gas-liquid two-phase mixture from being supplied, and thus the reliquefaction system can be smoothly controlled.

As described above, the embodiments according to the present invention have been reviewed, and the fact that the present invention can be embodied in other specific forms without departing from the spirit or scope of the present invention in addition to the above-described embodiments is recognized by those of ordinary skill in the art. It is self-evident to Therefore, the above-described embodiments are to be regarded as illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

100: LNG storage tank
200: heat exchanger
300: multi-stage compressor
510: first oil filter
520: second oil filter
610: first pressure reducing device
620: second pressure reducing device
630: pressure measuring unit
640: control unit
700: gas-liquid separator

Claims (11)

A compression step of compressing the boil-off gas discharged from the liquefied gas storage tank to the pressure required by the high-pressure engine;
a cooling step of cooling the high-pressure BOG compressed in the compression step by exchanging heat with BOG discharged from the liquefied gas storage tank and transferred to the compression step;
a first decompression step of first depressurizing the boil-off gas cooled in the cooling step; and
a second decompression step of further depressurizing the boil-off gas decompressed in the first decompression step to a final target pressure;
The first decompression step is,
In order to maintain a constant pressure of the boil-off gas transferred from the first decompression step to the second decompression step, a pressure control step of adjusting the degree of decompression of the boil-off gas in the first decompression step; Gas reliquefaction method. The method according to claim 1,
In the first decompression step and the second decompression step, the pressure of the boil-off gas is respectively performed by a pressure reducing valve, and the temperature of the boil-off gas is lowered while the pressure is reduced by the pressure reducing valve,
The pressure control step is
It is carried out by controlling the opening rate of the first pressure reducing valve for reducing the boil-off gas in the first pressure reducing step,
The opening rate of the second pressure reducing valve for decompressing the boil-off gas in the second decompression step is fixed, the method for reliquefying boil-off gas for ships. The method according to claim 1,
The pressure control step is
a pressure measuring step of measuring the pressure of the boil-off gas transferred from the first decompression step to the second decompression step; and
An opening rate adjusting step of adjusting the opening rate of the first pressure reducing valve for decompressing the boil-off gas in the first decompression step according to the pressure measurement value measured in the pressure measuring step; 4. The method according to claim 3,
In the step of adjusting the opening rate,
A larger value is obtained by comparing the basic opening rate corresponding to the intermediate target pressure targeted in the first decompression step and the variable opening rate required to increase or decrease the pressure measured value in the pressure measuring step to the intermediate target pressure. A method of re-liquefying BOG for ships, in which the opening rate of the first pressure reducing valve is set to the opening rate of . The method according to claim 1,
BOG supplied from the first decompression step to the second decompression step is in a gaseous state or a supercritical state, and the BOG decompressed in the second decompression step is in a liquid state or a gas-liquid mixed state. 6. The method of claim 5,
A recovery step of supplying the reliquefied BOG in the liquid state decompressed in the second decompression step to the liquefied gas storage tank; further comprising, a vessel BOG reliquefaction method. A compression step of compressing the boil-off gas discharged from the liquefied gas storage tank to the pressure required by the high-pressure engine;
a cooling step of cooling the high-pressure BOG compressed in the compression step by exchanging heat with BOG discharged from the liquefied gas storage tank and transferred to the compression step;
a first decompression step of first depressurizing the boil-off gas cooled in the cooling step; and
a second decompression step of further depressurizing the boil-off gas decompressed in the first decompression step to a final target pressure;
The first decompression step is,
a pressure control step of adjusting the degree of decompression of the boil-off gas in the first decompression step in order to prevent the state of the boil-off gas transferred from the first decompression step to a liquid state or a gas-liquid mixture state; A method of re-liquefaction of boil-off gas for ships, comprising 8. The method of claim 7,
The pressure control step is
a pressure measuring step of measuring the pressure of the boil-off gas transferred from the first decompression step to the second decompression step; and
adjusting an opening rate of a pressure reducing valve for decompressing boil-off gas in the first pressure reduction step according to the pressure measurement value of the pressure measurement step;
In the second decompression step, the opening rate of the pressure reducing valve for reducing the boil-off gas is a fixed value, the method for reliquefying boil-off gas for ships. a compressor for compressing the boil-off gas discharged from the liquefied gas storage tank to the pressure required by the high-pressure engine;
a heat exchanger configured to heat the high-pressure BOG compressed in the compressor with BOG discharged from the liquefied gas storage tank and transferred to the compressor to cool the high-pressure BOG;
a first pressure reducing device for first depressurizing the boil-off gas cooled in the heat exchanger;
a second pressure reducing device for further reducing the boil-off gas decompressed in the first pressure reducing device to a final target pressure;
a pressure measuring unit for measuring the pressure between the first pressure reducing device and the second pressure reducing device; and
A control unit for controlling the degree of decompression of the boil-off gas by the first pressure reducing device so that the pressure measurement value of the pressure measurement unit is maintained above a set value; 10. The method of claim 9,
The first pressure reducing device is a Joule-Thomson valve that expands the boil-off gas by an isenthalpy process,
The control unit is
By comparing the opening rate of the first pressure reducing device corresponding to the intermediate target pressure and the opening rate of the first pressure reducing device required to increase or decrease the pressure measurement value of the pressure measuring unit to the intermediate target pressure, the higher value is obtained. 1 High selector to set the opening rate of the pressure reducing device; including, BOG reliquefaction system for ships. 11. The method of claim 10,
The second pressure reducing device is a Joule-Thomson valve that expands the boil-off gas by an isenthalpy process,
The opening rate of the second pressure reducing device is a fixed value that does not change, BOG reliquefaction system for ships.

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