KR20210025150A - Boil-Off Gas Reliquefaction System and Method for Vessels - Google Patents

Abstract

The present invention relates to a system and a method for reliquefying evaporation gas of a ship which reliquefies evaporation gas generated by natural evaporation of liquefied gas.

Description

Boil-Off Gas Reliquefaction System and Method for Vessels}

The present invention relates to a boil-off gas re-liquefaction system and method of a ship for re-liquefying the boil-off gas generated by the natural vaporization of the liquefied gas.

Recently, the consumption of liquefied gas such as liquefied natural gas (LNG) 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 transport efficiency because the volume is very small compared to the gas. In addition, liquefied gas, including 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.

LNG is a colorless, transparent liquid that can be obtained by liquefying natural gas containing methane as its main component by cooling it to about -163°C, and has a volume of about 1/600 compared to natural gas.

As described above, the liquefaction temperature of natural gas is a cryogenic temperature of about -163°C at normal pressure, so LNG is sensitive to temperature changes and evaporates easily. For this reason, even if the storage tank for storing LNG is insulated, external heat is continuously transferred to the inside of the storage tank, so during the LNG transportation process, LNG is continuously evaporated in the storage tank and boil-off gas (BOG; Boil-) Off Gas) is created.

Boil-off gas is a loss, and the treatment of BOG is a very important issue in LNG transport efficiency. In addition, if boil-off gas is continuously generated and accumulated in the storage tank, the internal pressure of the storage tank may increase excessively, and there is a risk of damage. Therefore, various methods for efficiently treating the boil-off gas generated in the storage tank are being studied. Typically, for the treatment of boil-off gas, a method of re-liquefying boil-off gas and recovering it to a storage tank, and a method of using boil-off gas as a gas fuel of a customer, such as a ship's engine, are being applied.

The method of reliquefying the boil-off gas is a method of re-liquefying the boil-off gas by exchanging heat with the refrigerant by providing a refrigeration cycle using a separate refrigerant. There are methods and so on. In particular, a system employing the latter method is called a partial re-liquefaction system (PRS).

Among the engines generally used in ships, gas-fueled engines that can use natural gas as fuel include ME-GI (MAN Electronic Gas-Injection Engine) engines, X-DF (eXtra long stroke Dual Fuel) engines, and DF engines (DFDE). (Dual Fuel Diesel Electric), DFDG (Dual Fuel Diesel Generator, or DFGE)).

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 injected directly into the combustion chamber near the top dead center of the piston.

The X-DF engine consists of two strokes, uses 16 bar of natural gas as fuel, and operates on an auto cycle basis.

DFDE consists of four strokes, and operates based on the Otto Cycle, in which a relatively low pressure of natural gas with a pressure of about 6.5 bar is injected into the inlet of the combustion air, and the piston rises to compress it.

4 schematically shows a conventional partial reliquefaction system for a ship. The process of re-liquefying the boil-off gas by the conventional partial re-liquefaction system for ships is as follows.

As shown in Figure 4, according to the conventional boil-off gas reliquefaction system for ships, the boil-off gas discharged from the LNG storage tank (T) is first transferred to the heat exchanger (E). In the heat exchanger (E), the boil-off gas is used as a refrigerant to cool the compressed boil-off gas compressed in a later process, and then undergoes a multi-stage compression process by a multi-stage compressor (C) including a plurality of cylinders and a plurality of coolers. The boil-off gas compressed by the multi-stage compressor (C) is supplied to the fuel of the main engine, for example, the ME-GI engine. The remaining fuel is supplied as fuel of the ME-GI engine, and the remainder is transferred to the heat exchanger (E), and is cooled by heat exchange with the evaporated gas before compression.

As shown in FIG. 4, the boil-off gas compressed to a low pressure by some cylinders at the front end of the multi-stage compressor C may be supplied as fuel of a generator, for example, DFDE.

The boil-off gas cooled in the heat exchanger (E) is adiabatically expanded and reliquefied by the Joule-Thomson valve (V). The boil-off gas from the Joule-Thomson valve (V) is transferred to the gas-liquid separator (S) to separate the gas-liquid evaporated gas or flash gas that is not reliquefied, and the liquid re-liquefied evaporative gas from which the gas is separated is stored in LNG. The boil-off gas is recovered to the tank T, and the gaseous boil-off gas is joined to the pre-compression boil-off gas flow that is transferred to the heat exchanger (E).

On the other hand, when the boil-off gas is compressed at high pressure by the multi-stage compressor (C), in order to prevent abrasion of the compressor and extend its life, a cylinder of a lubrication type is generally used. As the boil-off gas is compressed by the oil supply lubrication type cylinder, a small amount of lubricating oil is mixed in the boil-off gas.

When the lubricating oil is mixed with the boil-off gas and supplied to the heat exchanger (E) and cooled, the condensation temperature of the lubricating oil is higher than the condensation temperature of the boil-off gas under the same pressure condition, so the lubricating oil phase changes to a solid or liquid state, and the lubricant has a higher viscosity. Is accumulated in the flow path of the heat exchanger. As this process is repeated, the flow path of the heat exchanger E is blocked.

The heat exchanger (E) applied to the partial reliquefaction system is generally a micro-channel type PCHE (printed circuit heat exchanger, also called DCHE).

As the compressed boil-off gas is cooled in the heat exchanger, the lubricating oil mixed with the compressed boil-off gas is condensed or solidified before the boil-off gas to block the flow path of the heat exchanger. In particular, in a PCHE type heat exchanger having a narrow flow path, for example, a micro-channel type flow path, a phenomenon in which the flow path of the heat exchanger is blocked by condensed or solidified lubricating oil occurs more frequently.

In addition, the PCHE type heat exchanger is a type that combines several heat exchanger blocks produced by stacking a plurality of heat exchange plates processed through chemical etching and performing diffusion bonding. The heat exchange efficiency is determined according to how evenly distributed and supplied the boil-off gas to be heat-exchanged through the formed flow path. When the flow path formed in any one heat exchanger block is blocked by lubricating oil, the flow rate imbalance becomes more severe and the heat exchange efficiency decreases.

As described above, in the case of the PCHE, due to its characteristics, the flow path is easily clogged by lubricating oil, and the flow channel clogging is a fatal factor deteriorating the heat exchange performance, such as causing an extreme flow rate imbalance.

In addition, when the reliquefied boil-off gas is recovered to the LNG storage tank (T) while the lubricant is mixed, the quality of the LNG stored in the LNG storage tank (T) is degraded, and the high-viscosity lubricant is used in the LNG storage tank (T). There is also a problem that damages.

In order to improve these problems, as shown in FIG. 4, an oil separator (OS) or an oil filter (OF) is installed in the line through which the boil-off gas compressed by the multi-stage compressor (C) is transferred to the heat exchanger (E). Thus, after the lubricating oil mixed with the boil-off gas is filtered, it is supplied to the heat exchanger (E), and a cryogenic filter (CF) is also installed between the expansion valve (V) and the gas-liquid separator (S), and is mixed with the reliquefied boil-off gas. A method of supplying the existing lubricating oil to the gas-liquid separator (S) after filtering it once more has been proposed.

However, as in the prior art, oil removal means such as an oil separator (OS), an oil filter (OF), and a cryogenic filter (CF) were installed, and further, 3 to 5 oil filters (OF) were continuously installed to take several steps. Even if the lubricating oil is filtered, the problem that the boil-off gas is reliquefied while the lubricating oil is still mixed with the boil-off gas is not solved.

Accordingly, in order to solve the above-described problem, the present invention can more reliably remove and re-liquefy the lubricating oil mixed in the high-pressure boil-off gas compressed by the multi-stage compressor in the system and method for partial re-liquefaction of the boil-off gas of the ship. It is intended to provide a system and method for reliquefaction of boil-off gas of a ship.

According to an aspect of the present invention for achieving the above object, the boil-off gas discharged from the liquefied gas storage tank is compressed above a supercritical pressure, which is a pressure required by a high-pressure engine, and includes at least one lubrication type cylinder. A multistage compressor including a plurality of cylinders and a plurality of intercoolers for cooling compressed boil-off gas discharged from each cylinder; A first decompression means for decompressing the supercritical boil-off gas compressed by the multi-stage compressor to a first pressure to phase change the lubricating oil mixed in the supercritical boil-off gas to a liquid state; An oil filter for filtering lubricating oil mixed in a liquid state in the boil-off gas by the first decompression means; And a heat exchanger that cools the boil-off gas filtered by the oil filter to the liquefaction temperature of the boil-off gas by heat exchange with the boil-off gas before compression before being compressed in the multi-stage compressor, wherein the first pressure is It is a pressure that satisfies any one or more of a condition below the pressure at which the lubricating oil is liquefied and a condition below the critical pressure of the boil-off gas, and the plurality of intercoolers are supercritical discharged from the last cylinder among the plurality of cylinders of the multistage compressor. An after-cooler for cooling the boil-off gas, wherein the after-cooler includes a supercritical boil-off gas discharged from the last cylinder and supplied to the high-pressure engine, and discharged from the last cylinder and supplied to the first decompression means. There is provided a boil-off gas reliquefaction system of a ship for cooling any one or more supercritical boil-off gas flows of the supercritical boil-off gas.

Preferably, a reliquefaction line connecting the last cylinder and the first decompression means, and flowing the supercritical boil-off gas passing through the last cylinder into the first decompression means from the upstream of the aftercooler; And a first fuel line connecting the aftercooler and the high-pressure engine, wherein the after-cooler cools the boil-off gas supplied to the high-pressure engine through the first fuel line to a temperature required by the high-pressure engine.

Preferably, the aftercooler is a multi-stage aftercooler, and includes two or more aftercoolers provided in series, and the two or more aftercoolers reduce the temperature of the boil-off gas while being depressurized by the first decompression means. A front end after-cooler cooling the lubricating oil to a temperature within a range in which the lubricating oil becomes a liquid state at the first pressure; And a rear-end after-cooler for cooling the boil-off gas cooled in the front-end after-cooler to a temperature required by the high-pressure engine, and connecting the front-end after-cooler and the first decompression means, and passing through the front-end after-cooler. And a re-liquefaction line for introducing supercritical boil-off gas into the first decompression means from the upstream of the after-cooler of the rear stage.

Preferably, the cooling load of the aftercooler is a fixed value, and the front aftercooler and the rear end so that the temperature of the boil-off gas cooled in the front aftercooler is within a range at which the lubricating oil is in a liquid state at the first pressure. It includes; a control unit for distributing and allocating the cooling load of the aftercooler, respectively.

Preferably, a second decompression means for reducing the boil-off gas cooled in the heat exchanger to an allowable pressure of the liquefied gas storage tank; further comprises.

Preferably, by gas-liquid separation of the boil-off gas depressurized by the second decompression means, the liquid re-liquefied boil-off gas is supplied to the liquefied gas storage tank, and the gaseous boil-off gas is transferred to the heat exchanger before compression. It further comprises a; gas-liquid separator for joining the boil-off gas flow.

According to another aspect of the present invention for achieving the above object, a plurality of cylinders including at least one lubrication type cylinder for boil-off gas discharged from the liquefied gas storage tank and compressed boil-off gas discharged from each of the cylinders A supercritical compression step of compressing above a supercritical pressure, which is a pressure required by a high-pressure engine, using a multistage compressor including a plurality of intercoolers to cool it; The supercritical boil-off gas compressed in the supercritical compression step is reduced to a first pressure that satisfies any one or more of a condition that is less than a pressure at which the lubricating oil is liquefied and a condition that is less than the critical pressure of the boil-off gas, and is mixed with the supercritical boil-off gas. A first depressurization step of phase-changing the existing lubricating oil into a liquid state; A liquid oil filtering step of filtering the lubricating oil mixed in a liquid state with the boil-off gas reduced in the first decompression step; And a cooling step of cooling the boil-off gas from which the lubricating oil is filtered in the liquid oil filtering step to the liquefaction temperature of the boil-off gas or lower by heat exchange with the boil-off gas before compression in the multi-stage compressor. The step is a temperature for cooling any one or more of the supercritical boil-off gas to be supplied from the last cylinder to the high-pressure engine among the plurality of cylinders and the supercritical boil-off gas to be supplied to the first decompression step from the last cylinder among the plurality of cylinders. A method of reliquefaction of boil-off gas of a ship is provided, including;

Preferably, in the temperature control step, the supercritical boil-off gas to be supplied to the first decompression stage is supplied to the first decompression stage without cooling, and the supercritical boil-off gas to be supplied to the high-pressure engine is cooled and supplied to the high-pressure engine. Gas fuel cooling step; may include.

Preferably, the temperature control step includes: a first cooling step of cooling the supercritical boil-off gas discharged from the last cylinder to a temperature required in the first decompression step; And a second cooling step of cooling the supercritical boil-off gas cooled in the first cooling step to a temperature required by the high-pressure engine, wherein the temperature required in the first depressurization step is a temperature required by the high-pressure engine. It can be a higher temperature.

Preferably, the total cooling load in the temperature adjustment step is a fixed value, and among the total cooling loads in the temperature adjustment step, a cooling load to be allocated to the first cooling step and a cooling load to be allocated to the second cooling step are selected. A load allocation step of distributing to each step; may further include.

Preferably, a second decompression step of reducing the boil-off gas cooled in the cooling step to an allowable pressure of the liquefied gas storage tank; may further include.

Preferably, by gas-liquid separation of the boil-off gas depressurized in the second decompression step, the liquid re-liquefied boil-off gas is supplied to the liquefied gas storage tank, and the gaseous boil-off gas is transferred to the cooling step before compression. It may further include a gas-liquid separation step of joining the boil-off gas flow.

The system and method for reliquefaction of boil-off gas of a ship according to the present invention can filter not only lubricating oil mixed in a liquid state and a vapor state in a compressed boil-off gas in a supercritical state, but also lubricating oil dissolved in the boil-off gas in a supercritical state. Therefore, the lubricant can be completely removed.

In addition, by completely removing the lubricating oil mixed in the compressed boil-off gas, it is possible to solve the problem of deterioration of heat exchange performance and contamination of the liquefied gas storage tank due to clogging of the flow path due to the lubricating oil or flow imbalance.

In addition, it is possible to reduce material cost because oil filters are installed in a minimum number.

1 is a schematic diagram of a system for reliquefaction of boil-off gas of a ship according to a first embodiment of the present invention.
2 is a schematic diagram of a system for reliquefaction of boil-off gas of a ship according to a second embodiment of the present invention.
3 is a schematic diagram of a system for reliquefaction of boil-off gas of a ship according to a third embodiment of the present invention.
4 is a schematic diagram of a partial reliquefaction system of a ship according to the prior art.
FIG. 5 is a PT diagram showing the phase change of lubricating oil according to the behavior of methane by overlapping the phase equilibrium line of methane and lubricating oil in the method for reliquefaction of boil-off gas of a ship according to the first embodiment of the present invention.
6 is a PT diagram showing a phase change of lubricating oil according to the behavior of methane by overlapping phase equilibrium lines of methane and lubricating oil in the method for reliquefaction of boil-off gas of a ship according to the second embodiment of the present invention.
7 is a PT diagram showing a phase change of lubricating oil according to the behavior of methane by overlapping phase equilibrium lines of methane and lubricating oil in the method for reliquefaction of boil-off gas of a ship according to the third embodiment of the present invention.
8 is a PT diagram showing a phase change of lubricating oil according to the behavior of methane by overlapping the phase equilibrium line of methane and lubricating oil in the method of partial reliquefaction of a ship according to the prior art.
In Figures 5 to 8, the phase equilibrium line of methane is represented by a normal solid line, and the phase equilibrium line of the lubricating oil is represented by a thick solid line.

In order to fully understand the operational advantages of the present invention and 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 a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Here, in adding reference numerals to elements of each drawing, it should be noted that only the same elements are marked with the same numerals as possible, even if they are indicated on different drawings. In addition, the following examples may be modified in various different forms, and the scope of the present invention is not limited to the following examples.

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

LNG is mainly composed of methane, and includes ethane, propane, butane, and the like, and its composition may vary depending on the production site.

In addition, in an embodiment of the present invention to be described later, the liquefied gas and the boil-off gas of the liquefied gas may be used as fuel for an engine, particularly an engine of a ship.

In addition, the boil-off gas treatment system and method according to an embodiment of the present invention to be described later will be described as an example applied to a ship, but may be applied on land.

In addition, in the embodiments 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 a cargo, but the present invention is an LNG FSRU (Floating FSRU) equipped with a storage tank for storing liquefied natural gas. Storage Regasification Unit), LNG FPSO (Floating Production Storage Offloading), LNG RV (Regasification Vessel), and other liquefied gas storage tanks are provided. Applicable to all ships equipped with a reliquefaction device for reliquefaction.

Accordingly, the present invention can be applied to all ships in which an engine receiving liquefied natural gas as a fuel is provided and capable of generating and using electric power by having a propulsive force by the engine or by driving the engine.

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 will be described as an example of an engine using gaseous fuel of about 100 bar to 400 bar, or about 150 bar or more, preferably about 300 bar, for example a ME-GI engine. Further, the medium pressure engine may be an engine using gaseous fuel of about 10 bar to 20 bar, preferably about 16 bar, for example an X-DF engine, and the low pressure engine is about 5 bar to 10 bar, preferably It may be an engine that uses about 6.5 bar of gaseous fuel, for example a DF engine or 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, including a ME-GI engine and a DFDE will be described as an example.

First, referring to FIG. 1, a system and method for re-liquefying boil-off gas of a ship according to a first embodiment of the present invention will be described.

A system for reliquefaction of boil-off gas of a ship according to a first embodiment of the present invention includes an LNG storage tank 100 for storing LNG; A multistage compressor 300 for compressing the boil-off gas transferred from the LNG storage tank 100 along the boil-off gas line BL to a high pressure equal to or higher than the supercritical pressure of the boil-off gas; The first to supply the remaining boil-off gas from the high-pressure boil-off gas compressed by the multistage compressor 300 to the engine and decompress the remaining boil-off gas to a first pressure that is less than the supercritical pressure of the boil-off gas and/or the pressure at which the lubricating oil phase changes to a liquid state. Decompression means 400; An oil filter 500 for filtering liquid lubricating oil mixed in the first decompression boil-off gas reduced to a first pressure P1 by the first decompression means 400; A heat exchanger 200 for cooling the primary decompression boil-off gas from which the lubricating oil is filtered by the oil filter 500 by heat exchange with the boil-off gas before compression that is transferred to the multistage compressor 300 along the boil-off gas line BL; A second decompression means 600 for decompressing the cooled boil-off gas cooled in the heat exchanger 200 to a second pressure P2 that is less than or equal to the allowable pressure of the LNG storage tank 100; And a gas-liquid separator 700 for gas-liquid separation of the reliquefied boil-off gas by the second decompression means 600 so that the re-liquefied boil-off gas in a liquid state from which the gas phase is separated is recovered to the LNG storage tank 100. .

In the LNG storage tank 100 of this embodiment, LNG may be stored at about 1 bar and -162°C. Boil-off gas (BOG) generated by natural vaporization of LNG in the LNG storage tank 100 is discharged from the LNG storage tank 100 and transported along the boil-off gas line BL, and the fuel lines FL1 and FL2 ) Is transported along and used as fuel for the engine, or reliquefied while being transported along the reliquefaction line (RL) and recovered back to the LNG storage tank (100).

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 a boil-off gas. It is transferred along the line BL, and after the cold heat is recovered from the heat exchanger 200, it is transferred to the multistage compressor 300.

The multistage compressor 300 may include a plurality of cylinders and a plurality of inter-coolers to compress the boil-off gas above a supercritical pressure over several steps.

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 the boil-off gas in at least one step to five steps is applied. Let's explain with an example.

Of the plurality of cylinders of the multistage compressor 300, at least one of the cylinders at the rear stage may include a cylinder of a lubrication type. In the present embodiment, it is assumed that the three cylinders at the rear end are cylinders of the lubrication system and the two cylinders at the front end are cylinders of the non-lubrication lubrication system. Therefore, there is no lubricating oil in the low-pressure boil-off gas compressed in two stages using only the two cylinders of the front, but the high-pressure compressed over five stages using both the two cylinders of the front and the three cylinders of the rear stage. Lubricating oil may be present in the boil-off gas.

The pressure of the low-pressure boil-off gas compressed in two stages by using two cylinders of an oil-free lubrication system at the front end of the multi-stage compressor 300 of this embodiment is a low-pressure engine, that is, the pressure of the gas fuel required by the DFDE in this embodiment. That is, it may be about 5 bar to 10 bar, or about 6.5 bar.

In addition, the pressure of the high-pressure boil-off gas compressed over five steps using both cylinders of the front end of the multistage compressor 300 and the three cylinders of the rear end of the lubrication lubrication system 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 the five stages in the multi-stage compressor 300 is a high-pressure engine, that is, about 150 bar or more, or about 100 bar to 400 bar, which is the gas fuel pressure required by the ME-GI engine in this embodiment. , Or about 300 bar.

According to the present embodiment, as shown in FIG. 1, among the low-pressure boil-off gas compressed over two stages by using some cylinders of the multi-stage compressor 300, that is, two cylinders of an oil-free lubrication system in this embodiment, The low-pressure boil-off gas corresponding to the amount of gas fuel required by the low-pressure engine can 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 multistage compressor 300 and is connected to the low-pressure engine.

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

The first fuel line FL1 connects the multistage compressor 300 and the high-pressure engine.

Among the high-pressure boil-off gas compressed over five stages by using all five cylinders of the multi-stage compressor 300, the remaining high-pressure boil-off gas excluding the amount of gas fuel required by the high-pressure engine is branched to the re-liquefaction line RL, and re-liquefied. It can be recovered to the LNG storage tank (100). The reliquefaction line RL is branched from the first fuel line FL1, and the boil-off gas is supplied with a first decompression means 400, an oil filter 500, a heat exchanger 200, a second decompression means 600, and a gas-liquid separator. It is connected to be recovered to the LNG storage tank 100 through 700.

The heat exchanger 200 of the present embodiment has a microchannel flow path, and may be a heat exchanger suitable for exchanging heat exchange between a cryogenic boil-off gas and a high-pressure boil-off gas, for example, a printed circuit heat exchanger (PCHE) type heat exchanger.

The reliquefaction line RL is branched from the first fuel line FL1 at the rear end of the fifth stage intercooler installed at the rear end of the fifth stage cylinder of the multistage compressor 300, and thus, the fifth stage cylinder of the multistage compressor 300 The supercritical boil-off gas after being compressed in and cooled in the fifth stage intercooler is branched into the reliquefaction line RL.

The pressure of the high-pressure boil-off gas introduced from the multi-stage compressor 300 to the reliquefaction line RL is higher than the supercritical pressure, and since it is the boil-off gas compressed by the lubricating cylinder of the multi-stage compressor 300, lubricating oil is mixed. .

In order to solve the clogging of the heat exchanger due to lubricating oil, a method of installing an oil filter or an oil remover upstream of the heat exchanger 200 has been proposed. However, even if the oil filter or oil remover is installed to filter out the lubricating oil mixed in the high-pressure boil-off gas before the high-pressure boil-off gas flows into the PCHE-type heat exchanger 200, the lubricating oil is not completely removed and the flow path of the heat exchanger is still blocked. Is not improving.

The inventors of the present invention have found that this problem occurs depending on the phase of the high-pressure boil-off gas and lubricating oil flowing from the multistage compressor 300 to the heat exchanger 200.

_{Methane (methane, CH 4} ), the main component of the boil-off gas, has a latent heat section below the critical pressure (about 47 bar in the case of pure methane). The latent heat section refers to a straight section where there is no change in temperature even with a change in the amount of heat flow.In the latent heat section, reliquefaction is not possible even when the temperature decreases. The reliquefaction efficiency is greatly reduced. Since the latent heat section does not appear in the supercritical fluid state due to the characteristics of methane, it is preferable in terms of reliquefaction efficiency and reliquefaction amount to reliquefy the boil-off gas in the supercritical fluid state.

Therefore, in order to increase the amount of reliquefaction, the higher the evaporation gas is, the more advantageous it is. The pressure of the high-pressure 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, preferably about 300 bar, as shown in the phase equilibrium diagram of FIG. It is in the first point (indicated by '1'), which is the status area.

That is, the supercritical boil-off gas compressed by the multi-stage compressor 300 is in the supercritical region (first point) on the P-T diagram of FIG. 5. At this time, the lubricating oil mixed in the supercritical evaporative gas discharged from the multistage compressor 300 is unconditionally generated above some vapor pressure due to the frictional heat in the liquid state and supercritical evaporation gas. It exists in a vaporized state.

The liquid lubricating oil mixed in the supercritical boil-off gas discharged from the multistage compressor 300 exists as very small droplets and is relatively easy to remove with an oil filter. However, the inventors of the present invention point that the liquid lubricant is well filtered by the oil filter or oil separator, but the lubricant in the vapor state and the lubricating oil dissolved in the supercritical evaporation gas are not filtered and remain. Found.

In the existing reliquefaction system shown in FIG. 4, in order to remove the lubricating oil mixed in the supercritical evaporation gas discharged from the multistage compressor (C), one oil separator (OS) and two to three coarles. Although several lubricating oil removal filters were installed, such as a coalesce type oil filter (OF) and one or two cryogenic filters (CF), it can filter only liquid lubricating oil components. In reality, the lubricating oil in the vapor state mixed with the evaporation gas in the supercritical state and the dissolved lubricating oil cannot be filtered out realistically, so the lubricating oil is collected as it is in the LNG storage tank (T).

To solve this, in the present invention, as shown in FIG. 1, the pressure of the boil-off gas mixed with the lubricating oil, introduced into the reliquefaction line RL from the multi-stage compressor 300, is dissolved in the boil-off gas in a supercritical state. Between the first decompression means 400 and the first decompression means 400 and the heat exchanger 200 for reducing the pressure of the lubricating oil and the lubricating oil in a vapor state to a phase change to a liquid state, that is, to a pressure equal to or less than the first pressure P1 It is installed in, and further includes an oil filter 500 for filtering the lubricating oil mixed in a liquid phase in the evaporation gas while being depressurized by the first decompression means 400.

That is, the high-pressure boil-off gas to be reliquefied is depressurized using the first decompression means 400 so that the lubricating oil mixed in the boil-off gas quickly leaves the supercritical region, and all phases are changed to a liquid state, and then the liquid lubricating oil is The liquid lubricating oil is filtered using an oil filter 500 suitable for filtering, and the boil-off gas from which the lubricating oil has been filtered is supplied to the heat exchanger 200 for cooling.

At this time, since the oil filter 500 filters the lubricating oil in a state in which all of the lubricating oil is mixed in a liquid phase with the boil-off gas, it is possible to completely filter the lubricating oil from the boil-off gas with only a minimum number of them.

The first pressure reducing means 400 may be an expansion valve such as a Joule-Thomson valve, or may be an expander. In this embodiment, the first pressure reducing means 400 will be described as an example that is a Joule-Thomson valve.

Referring to FIG. 5, the pressure of the boil-off gas reduced by the first decompression means 400 is lowered to a second point, that is, a liquid region of the lubricating oil. When the pressure of the boil-off gas is lowered to the first pressure by the first decompression means 400, the lubricating oil is in the liquid region, so that the gaseous lubricating oil mixed with the supercritical boil-off gas and the supercritical boil-off gas are dissolved. All of the lubricants change phase to liquid state.

However, the supercritical boil-off gas (methane) rapidly decreases in pressure and temperature by the first decompression means 400, and is in the gas phase region out of the supercritical region, so that the phase can be changed into a gaseous state. That is, the lubricating oil exists in a liquid state in gaseous methane, and the lubricating oil may be separated by the oil filter 500 capable of separating droplets.

Of the supercritical boil-off gas compressed by the multi-stage compressor 300 (the first point in FIG. 5), the boil-off gas to be reliquefied is reduced to a first pressure by the first decompression means 400 (the first point in FIG. 5). 2), all of the lubricating oil mixed in the boil-off gas is phase-changed to a liquid state, and all of the lubricating oil is filtered by the oil filter 500 for filtering droplets, and then supplied to the heat exchanger 200.

The high-pressure boil-off gas from which the lubricating oil is filtered is cooled by heat exchange with the boil-off gas before compression in the heat exchanger (200). In the heat exchanger 200, the high-pressure boil-off gas may be lowered to a temperature below the liquefaction temperature of methane (a third point in FIG. 5).

The boil-off gas liquefied by heat exchange in the heat exchanger 200 is reduced to the second pressure P2 by the second decompression means 600.

The second pressure reducing means 600 may be an expansion valve such as a Joule-Thomson valve, or may be an expander. In this embodiment, the second pressure reducing means 600 will be described as an example that the Joule-Thomson valve.

The second decompression means 600 decompresses the boil-off gas cooled in the heat exchanger 200 to a second pressure, and the second pressure is equal to or less than the allowable pressure of the LNG storage tank 100 (the fourth point in FIG. 5 ). I can. That is, in this embodiment, LNG is stored in the LNG storage tank 100 at a pressure of about 1 bar, and the LNG storage tank 100 generally allows a slight pressure increase, so the second pressure is about 1 bar to It can be 1.5 bar.

As the boil-off gas is reduced to a second pressure by the second decompression means 600, flash gas or the like may be generated.

The boil-off gas depressurized by the second decompression means 600 is gas-liquid separated by the gas-liquid separator 700, and the liquid re-liquefied boil-off gas is recovered to the LNG storage tank 100 along the re-liquefaction line RL. In addition, the vaporized gas in a gaseous state separated from the gas-liquid separator 700 may be joined to the flow of the vaporized gas before compression supplied to the heat exchanger 200 along the gas recovery line GL.

The gas recovery line GL connects the gas-liquid separator 700 and the boil-off gas line BL, and joins the boil-off gas line BL upstream of the heat exchanger 200.

The lubricating oil used in the lubrication type cylinder of the multi-stage compressor 300 has a slightly different component for each supplier, and thus the freezing point of the lubricating oil varies from about -60°C to -20°C.

Referring to FIG. 7, the supercritical boil-off gas compressed by the multi-stage compressor 300 (point 1 ``in FIG. 7) is a first pressure that is less than the supercritical pressure of the boil-off gas by the first decompression means 400 When the pressure is reduced to (P1) (the second ``point in Fig. 7), depending on the components of the lubricant, the lubricating oil may reach a freezing point or less at which the lubricating oil phase changes to a solid state. In this case, there may be a case where the lubricating oil phase changes to a solid state, and the viscosity increases and accumulates in the pipe.

That is, as the supercritical boil-off gas compressed in the multi-stage compressor 300 is depressurized by the first decompression means 400, it is lowered to the first pressure P1 and the tenth temperature T10, but at the first pressure P1 Since the tenth temperature T10 corresponds to the solid region of the lubricating oil, when the lubricating oil is mixed with the boil-off gas in a solid state and flows into the oil filter 500, the problem of deteriorating the performance of the oil filter 500 dedicated to droplet separation Causes, and it becomes very difficult to collect the lubricating oil.

Accordingly, the second and third embodiments to be described later are characterized in that the pressure and temperature of the boil-off gas lowered while being depressurized by the first decompression means 400 do not correspond to the solid region of the lubricating oil, but are in the liquid region. It is done.

First, with reference to FIGS. 2 and 6, a description will be made of a system and method for re-liquefying boil-off gas of a ship according to a second embodiment of the present invention.

The second embodiment of the present invention is a modified example of the first embodiment, wherein the reliquefaction line RL is branched between the fifth stage cylinder 309 and the fifth stage intercooler 310, and the fifth stage cylinder 309 ), the high-temperature supercritical boil-off gas compressed in the fifth stage intercooler 310 flows into the reliquefaction line RL before flowing into the fifth stage intercooler 310, and the gaseous fuel to be supplied to the high-pressure engine is flown along the first fuel line FL1. However, there is a difference in that it is cooled in the intercooler 310 and then supplied as gaseous fuel of a high-pressure engine. Accordingly, differences from the first embodiment will be mainly described, and detailed descriptions of the same configuration will be omitted. Even if a detailed description is omitted, the same can be applied to the first embodiment described above.

That is, according to the present embodiment, in order to ensure that the pressure and temperature of the boil-off gas depressurized by the first decompression means 400 is not lower than the freezing point of the lubricating oil, and is in a liquid state, the fifth stage cylinder 309 is Before the supercritical boil-off gas compressed by the fifth stage intercooler 310, that is, the after-cooler 310, is cooled, the boil-off gas to be re-liquefied is branched from the upstream of the after-cooler 310, and the first implementation is carried out. It is characterized in that the supercritical boil-off gas is depressurized by the first decompression means 400 at a higher temperature than before.

The boil-off gas reliquefaction system of a ship according to this embodiment includes an LNG storage tank 100 for storing LNG; And a multi-stage compressor 300 for compressing the boil-off gas transferred along the boil-off gas line BL from the LNG storage tank 100 to a high pressure equal to or higher than the supercritical pressure of the boil-off gas.

Here, the multi-stage compressor 300 of the present embodiment includes an after cooler 310 for cooling the boil-off gas to be supplied to the high-pressure engine among the high-temperature supercritical boil-off gas compressed by the last cylinder 309 among a plurality of cylinders.

In addition, the boil-off gas re-liquefaction system of the ship according to the present embodiment, from the high-temperature supercritical boil-off gas compressed by the last cylinder 309, as much as the fuel demand required by the high-pressure engine supplied to the aftercooler 310. A first decompression means 400 for decompressing the remaining boil-off gas to a first pressure that is less than the supercritical pressure of the boil-off gas and/or the pressure at which the lubricating oil phase changes to a liquid state; And an oil filter 500 that is depressurized to a first pressure by the first decompression means 400 and cooled to a fourth temperature T4 to filter lubricating oil from the boil-off gas in which all remaining lubricating oil components are mixed in a liquid state. A heat exchanger 200 for cooling the boil-off gas from which the lubricating oil is filtered by the oil filter 500 by heat exchange with the boil-off gas before compression that is transferred to the multi-stage compressor 300 along the boil-off gas line BL; A second decompression means 600 for decompressing the cooled boil-off gas cooled in the heat exchanger 200 to a second pressure equal to or less than the allowable pressure of the LNG storage tank 100; And a gas-liquid separator 700 for gas-liquid separation of the reliquefied boil-off gas by the second decompression means 600 so that the re-liquefied boil-off gas in a liquid state from which the gas phase is separated is recovered to the LNG storage tank 100. .

The multi-stage compressor 300 of this embodiment may be a five-stage compressor, as described in the first embodiment described above, and includes a first stage cylinder 301 and a second stage cylinder 303 of an oil-free lubrication system, It includes a third-stage cylinder 305, a fourth-stage cylinder 307, and a fifth-stage cylinder 309 of a lubrication type.

After the cold heat is recovered from the heat exchanger 200, the boil-off gas supplied to the multistage compressor 300 is first compressed by a first stage cylinder 301 and compressed by a first stage cylinder 301. However, the compressed boil-off gas is compressed in two stages by the second stage cylinder 303. At this time, the first-stage compressed boil-off gas compressed by the first-stage cylinder 301 is cooled by the first-stage intercooler 302 before being compressed by the second-stage cylinder 303 to increase the density, and then the second-stage cylinder Compressed by 303.

The second-stage compressed boil-off gas compressed by the first stage cylinder 301, cooled by the first stage intercooler 302 and compressed by the second stage cylinder 303 is 3 by the third stage cylinder 305. Step is compressed. Similarly, the two-stage compressed boil-off gas compressed in two stages by the first stage cylinder 301 and the second stage cylinder 302 is compressed by the second stage intercooler 304 before being compressed by the third stage cylinder 305. After cooling and increasing density, it is compressed by the third stage cylinder 305.

The third-stage compressed boil-off gas compressed while passing through the first-stage cylinder 301, first-stage intercooler 302, second-stage cylinder 303, second-stage intercooler 304, and third-stage cylinder 305 is It is compressed in four stages by the fourth stage cylinder 305. Similarly, the three-stage compressed boil-off gas compressed three stages by the first stage cylinder 301, the second stage cylinder 303, and the third stage cylinder 305 is removed before being compressed by the fourth stage cylinder 307. It is cooled by the three-stage intercooler 306 to increase the density and then compressed by the fourth-stage cylinder 307.

First stage cylinder 301, first stage intercooler 302, second stage cylinder 303, second stage intercooler 304, third stage cylinder 305, third stage intercooler 306 and fourth stage However, the four-stage compressed boil-off gas compressed while passing through the cylinder 307 is compressed in five stages by the fifth-stage cylinder 309. Similarly, the four-stage compressed boil-off gas compressed four stages by the first stage cylinder 301, the second stage cylinder 303, the third stage cylinder 305 and the fourth stage cylinder 307 is the fifth stage cylinder ( It is cooled by the fourth stage intercooler 308 before being compressed by 309 to increase the density, and then compressed by the fifth stage cylinder 309.

First stage cylinder 301, first stage intercooler 302, second stage cylinder 303, second stage intercooler 304, third stage cylinder 305, third stage intercooler 306, fourth stage However, the fifth-stage compressed boil-off gas compressed while passing through the cylinder 307, the fourth-stage intercooler 308, and the fifth-stage cylinder 309 is in a supercritical state (the first point in FIG. 6).

The five-stage compressed boil-off gas diverges the flow into boil-off gas equal to the amount of fuel demanded by the high-pressure engine and the remaining boil-off gas exceeding the amount of fuel demanded by the high-pressure engine.

That is, the 5-stage compressed boil-off gas is supplied by dividing into the first fuel line FL1 and the reliquefaction line RL. The boil-off gas equivalent to the fuel demand required by the high-pressure engine is supplied to the first fuel line FL1, and among the five-stage compressed boil-off gas, the remaining boil-off gas exceeding the fuel demand required by the high-pressure engine, that is, the boil-off gas for reliquefaction Is supplied to the reliquefaction line RL.

In the first fuel line FL1, an aftercooler 310, which is a final intercooler that increases density by cooling the compressed boil-off gas compressed in the fifth-stage cylinder 309, is installed, and branches to the first fuel line FL1. The high-temperature five-stage compressed boil-off gas is cooled in the after cooler 310 to increase the density, and then supplied as fuel of the high-pressure engine.

In addition, the reliquefaction line RL is branched between the fifth stage cylinder 309 and the aftercooler 310, and the supercritical boil-off gas supplied to the reliquefaction line RL is compressed in the fifth stage cylinder 309. It is a high-temperature 5-stage compressed evaporation gas before being cooled in the aftercooler 310 after being cooled.

Referring to FIG. 7, in this embodiment, the temperature of the supercritical boil-off gas supplied to the reliquefaction line RL is a third temperature T3, which is supplied to the reliquefaction line RL in the above-described first embodiment. It is higher than the first temperature T1, which is the temperature of the supercritical boil-off gas.

The first decompression means 400 of this embodiment decompresses the supercritical boil-off gas of the third temperature T3 to the first pressure P1, and in this process, the temperature of the boil-off gas is lowered to the fourth temperature T4.

That is, in the fluid (the second point in FIG. 6) whose pressure and temperature are lowered while passing through the first decompression means 400, the boil-off gas is present in a gaseous state, and the lubricating oil is present in a liquid state.

The gas-liquid mixture formed by the first decompression means 400 is supplied to the oil filter 500.

The oil filter 500 is a filter member optimized for separating droplets from a gas, and a liquid lubricating oil is filtered from a gaseous boil-off gas while passing through the oil filter 500.

The gaseous boil-off gas from which the liquid lubricating oil is filtered in the oil filter 500 is cooled by heat exchange with the boil-off gas before compression in the heat exchanger 200 (the third point in FIG. 6), and the second decompression means 600 It is reduced to the second pressure by the (4th point in Fig. 6).

The boil-off gas depressurized by the second decompression means 600 is gas-liquid separated by a gas-liquid separator 700, and the liquid re-liquefied boil-off gas is recovered to the LNG storage tank 100, and the gaseous boil-off gas is gas. It is joined to the flow of boil-off gas before compression supplied to the heat exchanger 200 along the recovery line GL.

Meanwhile, among the two-stage compressed boil-off gas cooled by the second-stage intercooler 303, the boil-off gas exceeding the fuel demand of the high-pressure engine is branched to the second fuel line FL2 by the amount of gas-fuel demand required by the low-pressure engine. Can be.

Next, referring to FIGS. 3 and 7, a system and method for re-liquefying boil-off gas of a ship according to a third embodiment of the present invention will be described.

The third embodiment of the present invention is a modified example of the second embodiment, and unlike the second embodiment in which the aftercooler 310 is configured in one stage, this embodiment has an aftercooler in multiple stages, and is reliquefied. The line RL is branched downstream of at least one aftercooler 311 among the multistage aftercoolers 311 and 312, and the supercritical boil-off gas cooled over one or more stages by the multistage aftercoolers 311 and 312 is regenerated. There is a difference in that it is supplied to the liquefaction line RL. Accordingly, the description of the present embodiment will focus on differences from the second embodiment described above, and detailed descriptions of the same configuration will be omitted. Even if a detailed description is omitted, the same can be applied to the second embodiment described above.

The boil-off gas reliquefaction system of a ship according to this embodiment includes an LNG storage tank 100 for storing LNG; And a multi-stage compressor 300 for compressing the boil-off gas transferred along the boil-off gas line BL from the LNG storage tank 100 to a high pressure equal to or higher than the supercritical pressure of the boil-off gas.

Here, the multi-stage compressor 300 of the present embodiment is a multi-stage aftercooler 311, 312 for cooling the boil-off gas to be supplied to the high-pressure engine among the high-temperature supercritical boil-off gas compressed by the last cylinder 309 among a plurality of cylinders; Includes.

In this embodiment, the multi-stage after-cooler 311 and 312 includes a first-stage after-cooler 311 and a second-stage after-cooler 312, and can cool the 5-stage compressed boil-off gas over two stages. However, it may be the after coolers 311 and 312. However, the present invention is not limited thereto, and the multi-stage aftercooler 311 and 312 may be a three-stage aftercooler composed of three aftercoolers.

The multi-stage aftercooler 311, 312 cools the fifth stage compressed boil-off gas compressed in the last cylinder, that is, the fifth stage cylinder 309 in this embodiment, through multiple stages. The final cooling target temperature, that is, the temperature of the 5-stage compressed boil-off gas cooled and discharged from the aftercooler of the last stage, for example, the second stage aftercooler 312 of the present embodiment, is the temperature of the gas fuel required by the high-pressure engine. I can.

First stage cylinder 301, first stage intercooler 302, second stage cylinder 303, second stage intercooler 304, third stage cylinder 305, third stage intercooler 306 of this embodiment , The fifth-stage compressed boil-off gas compressed while passing through the fourth-stage cylinder 307, the fourth-stage intercooler 308, and the fifth-stage cylinder 309 is within the pressure range of gas fuel required by the high-pressure engine, or during transfer. It may be a slightly higher pressure, taking into account the pressure loss, and is in a supercritical state (point 1 in FIG. 7).

The five-stage compressed boil-off gas of this embodiment, that is, the supercritical boil-off gas compressed to the pressure required by the high-pressure engine using the multi-stage compressor 300, is cooled by the first-stage aftercooler 311, and then in the high-pressure engine. It diverges into a boil-off gas flow equal to the required fuel demand and the remaining boil-off gas flow exceeding the fuel demand required by the high-pressure engine.

Since the temperature of the boil-off gas fuel supplied to the high-pressure engine is fixed, the cooling load of the multi-stage aftercoolers 311 and 312 is always constant. According to the present embodiment, a plurality of aftercoolers 311 and 312 are used to divide the cooling load of this predetermined value into multiple stages and allocate it.

The reliquefaction line RL of this embodiment is branched between the first stage aftercooler 311 and the second stage aftercooler 312.

Among the supercritical boil-off gas cooled by the first-stage aftercooler 311, the boil-off gas equivalent to the fuel demand required by the high-pressure engine is supplied to the first fuel line FL1, and exceeds the fuel demand required by the high-pressure engine. The remaining boil-off gas, that is, the boil-off gas to be reliquefied, is supplied to the re-liquefaction line RL.

In the first fuel line FL1, a second-stage after-cooler 312 for further cooling the supercritical boil-off gas cooled by the first-stage after-cooler 311 to a temperature required by the high-pressure engine is installed, and the first The high-temperature 5-stage compressed boil-off gas branched to the fuel line FL1 is further cooled by the second-stage aftercooler 312 to increase the density and then supplied as fuel of the high-pressure engine.

In addition, the reliquefaction line RL is branched between the fifth stage cylinder 309 and the aftercooler 310, and the supercritical boil-off gas supplied to the reliquefaction line RL is compressed in the fifth stage cylinder 309. It is a 5-stage compressed boil-off gas that has been pre-cooled in the first-stage after-cooler 311 after being cooled down.

Referring to FIG. 7, the temperature of the supercritical boil-off gas supplied to the reliquefaction line RL in this embodiment is a fifth temperature T5, which is supplied to the reliquefaction line RL in the above-described first embodiment. It is higher than the first temperature T1, which is the temperature of the supercritical boil-off gas, and is lower than the third temperature T3, which is the temperature of the supercritical boil-off gas supplied to the reliquefaction line RL in the second embodiment.

The first decompression means 400 of the present embodiment decompresses the supercritical boil-off gas of the fifth temperature T5 to the first pressure P1, and in this process, the temperature of the boil-off gas is lowered to the sixth temperature T6.

That is, in the fluid (the second point in FIG. 7) whose pressure and temperature are lowered while passing through the first decompression means 400, the boil-off gas is present in a gaseous state and the lubricating oil is present in a liquid state.

The gas-liquid mixture formed by the first decompression means 400 is supplied to the oil filter 500.

The oil filter 500 is a filter member optimized for separating droplets from a gas, and a liquid lubricating oil is filtered from a gaseous boil-off gas while passing through the oil filter 500.

The gaseous boil-off gas from which the liquid lubricating oil is filtered in the oil filter 500 is heat-exchanged with the boil-off gas before compression in the heat exchanger 200 and cooled (the third point in FIG. 7), and the second decompression means 600 It is reduced to the second pressure by the (4th point in Fig. 7).

The boil-off gas depressurized by the second decompression means 600 is gas-liquid separated by a gas-liquid separator 700, and the liquid re-liquefied boil-off gas is recovered to the LNG storage tank 100, and the gaseous boil-off gas is gas. It is joined to the flow of boil-off gas before compression supplied to the heat exchanger 200 along the recovery line GL.

Meanwhile, among the two-stage compressed boil-off gas cooled by the second-stage intercooler 303, the boil-off gas exceeding the fuel demand of the high-pressure engine is branched to the second fuel line FL2 by the amount of gas-fuel demand required by the low-pressure engine. Can be.

According to this embodiment, in order to ensure that the pressure and temperature of the boil-off gas depressurized by the first decompression means 400 is not lower than the freezing point of the lubricating oil, and all become liquid, a plurality of multi-stage aftercoolers 311 and 312 The loads of the three aftercoolers 311 and 312 are allocated differently and controlled. Accordingly, by optimizing the temperature of the supercritical boil-off gas to be reliquefied and supplied to the first decompression unit 400 after being cooled in the first stage after-cooler 311, the evaporation reduced by the first decompression unit 400 Make sure that the lubricating oil mixed with the gas does not become a solid state or remains in a gaseous state, but is completely in a liquid state.

For example, when the cooling load of the multi-stage aftercooler 311 and 312 is 100, the cooling load of the first-stage aftercooler 311 is assigned to 10, and the cooling load of the second-stage aftercooler 312 is When assigned to 90, the temperature of the boil-off gas cooled by the first-stage aftercooler 311 may be higher than the seventh temperature T7, that is, the fifth temperature T5, which is a target appropriate temperature.

Here, the target appropriate temperature means that when the boil-off gas cooled in the first stage aftercooler 311 is depressurized to the first pressure P1 by the first decompression means 400, the pressure and temperature are at which the lubricating oil becomes a liquid. This is the temperature that keeps you in the temperature range.

When the boil-off gas of the seventh temperature T7 is reduced to the first pressure P1 by the first decompression means 400, it is lowered to the eighth temperature T8, the first pressure P1 and the eighth temperature ( Since the lubricating oil of T8) is in a gaseous state, the lubricating oil is not filtered even if it passes through the oil filter 500.

On the other hand, when the cooling load of the multi-stage aftercoolers 311 and 312 is 100, the cooling load of the first stage aftercooler 311 is assigned to 70, and the cooling load of the second stage aftercooler 312 is set to 30. In the case of allocation, the temperature of the boil-off gas cooled by the first-stage aftercooler 311 may be lower than the ninth temperature T9, that is, the fifth temperature T5, which is a target appropriate temperature.

When the boil-off gas of the ninth temperature (T5) is reduced to the first pressure (P1) by the first pressure reducing means (400), it is lowered to the tenth temperature (T10), the first pressure (P1), the tenth temperature ( Since the lubricating oil of T10) is in a solid state, the lubricating oil is not filtered even if it passes through the oil filter 500.

That is, if the temperature of the boil-off gas supplied to the first decompression means 400 is higher than the appropriate temperature, even if the pressure is reduced by the first decompression means 400, the lubricating oil does not change into a liquid state and remains in a gaseous state. If the temperature of the boil-off gas supplied to the means 400 is lower than the appropriate temperature, the lubricating oil changes to a solid state when the pressure is reduced by the first decompression means 400, so even if the oil filter 500 is installed, the heat exchanger 200 Lubricating oil remains in the boil-off gas supplied to the furnace.

Therefore, the controller (not shown) controls the cooling load allocation amount of the first-stage aftercooler 311 and the second-stage aftercooler 312 according to the operating state of the ship, and is cooled in the first-stage aftercooler 311. The temperature of the boil-off gas supplied to the first decompression means 400 may be controlled to be an appropriate temperature.

As described above, according to the present invention, all of the liquid lubricating oil mixed with the supercritical evaporative gas compressed in the multistage compressor 300, the vaporized lubricating oil, and the lubricating oil dissolved in the supercritical evaporative gas A first pressure reducing device 400 is provided as a lubricating oil phase change means for liquefying, and the temperature of the supercritical boil-off gas supplied to the first pressure reducing device 400 is first reduced by using the after coolers 310, 311, and 312. After depressurization by the device 400, the temperature of the lubricating oil mixed in the boil-off gas is controlled to an appropriate temperature so that the lubricating oil mixed in the boil-off gas becomes a liquid state. It does not remain in a gaseous state and can be made to liquefy all.

After liquefying all the lubricating oil mixed in the boil-off gas, the lubricating oil in the liquid state is filtered by the oil filter 500, so that the lubricating oil contained in the boil-off gas to be reliquefied can be completely removed.

In addition, by converting all the state of the lubricating oil mixed in the boil-off gas into a liquid state and filtering it with an oil filter, the lubricating oil can be effectively removed with only a minimum oil filter, so that material cost can be reduced.

In addition, by removing the boil-off gas after removing the lubricating oil mixed in the boil-off gas to be re-liquefied, it is possible to prevent clogging of the flow path of the heat exchanger and the flow rate imbalance, thereby improving heat exchange performance and re-liquefaction performance.

In addition, since the lubricating oil is not mixed with the re-liquefied boil-off gas and recovered to the LNG storage tank, contamination or damage to the LNG storage tank can be prevented.

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

100: LNG storage tank
200: heat exchanger
300: multi-stage compressor
310: after cooler
311: 1st stage after cooler
312: 2nd stage after cooler
400: first decompression means
500: oil filter
600: second decompression means
700: gas-liquid separator
BL: Boil-off gas line
FL1: first fuel line
FL2: second fuel line
RL: Reliquefaction line
GL: Gas recovery line

Claims (12)

Compresses the boil-off gas discharged from the liquefied gas storage tank above the supercritical pressure, which is the pressure required by the high-pressure engine, and contains a plurality of cylinders including at least one oil supply lubricating cylinder, and compressed boil-off gas discharged from each cylinder. A multistage compressor including a plurality of intercoolers to cool;
A first decompression means for decompressing the supercritical boil-off gas compressed by the multi-stage compressor to a first pressure to phase change the lubricating oil mixed in the supercritical boil-off gas to a liquid state;
An oil filter for filtering lubricating oil mixed in a liquid state in the boil-off gas by the first decompression means; And
A heat exchanger for cooling the boil-off gas filtered by the oil filter to the liquefaction temperature of the boil-off gas by heat exchange with the boil-off gas before compression before being compressed in the multi-stage compressor; and
The first pressure is a pressure that satisfies any one or more of a condition below a pressure at which the lubricating oil is liquefied and a condition below a critical pressure of the boil-off gas,
The plurality of intercoolers include an after cooler for cooling the supercritical boil-off gas discharged from the last cylinder among the plurality of cylinders of the multi-stage compressor,
The after-cooler, at least one of supercritical evaporation gas discharged from the last cylinder and supplied to the high pressure engine, and supercritical evaporation gas discharged from the last cylinder and supplied to the first decompression means. A ship's boil-off gas reliquefaction system that cools the gas stream. The method according to claim 1,
A reliquefaction line connecting the last-stage cylinder and the first decompression means, and flowing the supercritical boil-off gas that has passed through the last-stage cylinder from the upstream of the aftercooler to the first decompression means; And
A first fuel line connecting the aftercooler and the high-pressure engine; further includes,
The aftercooler cools the boil-off gas supplied to the high-pressure engine through the first fuel line to a temperature required by the high-pressure engine. The method according to claim 1,
The aftercooler is a multistage aftercooler, and includes two or more aftercoolers provided in series,
The two or more aftercoolers,
A front end aftercooler for cooling the temperature of the boil-off gas lowered while being reduced by the first decompression means to a temperature within a range in which the lubricating oil becomes a liquid state at the first pressure; And
Includes; a rear-stage after-cooler for cooling the boil-off gas cooled in the front-end after-cooler to a temperature required by the high-pressure engine; and
A reliquefaction line that connects the front after cooler and the first decompression means, and flows the supercritical boil-off gas that has passed through the front after cooler to the first decompression means from the upstream of the rear after cooler. Boil-off gas reliquefaction system. The method of claim 3,
The cooling load of the aftercooler is a fixed value,
A control unit for distributing and allocating cooling loads of the front after cooler and the rear after cooler so that the temperature of the boil-off gas cooled by the front after cooler is within a range in which the lubricating oil is in a liquid state at the first pressure; That, the ship's boil-off gas reliquefaction system. The method according to any one of claims 1 to 4,
A second decompression means for decompressing the boil-off gas cooled in the heat exchanger to an allowable pressure of the liquefied gas storage tank; further comprising, a boil-off gas reliquefaction system of a ship. The method of claim 5,
By gas-liquid separating the boil-off gas depressurized by the second decompression means, the re-liquefied boil-off gas in a liquid state is supplied to the liquefied gas storage tank, and the boil-off gas in the gaseous state is transferred to the pre-compression boil-off gas flow to the heat exchanger. Gas-liquid separator for confluence; further comprising, the boil-off gas reliquefaction system of the ship. A high-pressure engine using a multi-stage compressor including a plurality of cylinders including at least one oil-lubricating cylinder for boil-off gas discharged from the liquefied gas storage tank and a plurality of intercoolers for cooling the compressed boil-off gas discharged from each of the cylinders A supercritical compression step of compressing above the supercritical pressure, which is the pressure required by the system;
The supercritical boil-off gas compressed in the supercritical compression step is reduced to a first pressure that satisfies any one or more of a condition that is less than a pressure at which the lubricating oil is liquefied and a condition that is less than the critical pressure of the boil-off gas, and is mixed with the supercritical boil-off gas. A first decompression step of phase-changing the lubricating oil into a liquid state;
A liquid oil filtering step of filtering the lubricating oil mixed in a liquid state with the boil-off gas reduced in the first decompression step; And
Including; a cooling step of cooling the boil-off gas from which the lubricating oil is filtered in the liquid oil filtering step to the liquefaction temperature of the boil-off gas by heat exchange with the boil-off gas before compression before being compressed in the multi-stage compressor; and
In the supercritical compression step, any one or more flows of a supercritical boil-off gas to be supplied from the last cylinder among the plurality of cylinders to the high-pressure engine and the supercritical boil-off gas to be supplied from the last cylinder of the plurality of cylinders to the first decompression stage Including a temperature control step of cooling the boil-off gas reliquefaction method of the ship. The method of claim 7,
The temperature control step,
Including; a gas fuel cooling step of supplying the supercritical boil-off gas to be supplied to the first decompression step to the first decompression step without cooling, and cooling the supercritical boil-off gas to be supplied to the high-pressure engine and supplying it to the high-pressure engine; Method of reliquefaction of boil-off gas. The method of claim 7,
The temperature control step,
A first cooling step of cooling the supercritical boil-off gas discharged from the last cylinder to a temperature required in the first decompression step; And
A second cooling step of cooling the supercritical boil-off gas cooled in the first cooling step to a temperature required by the high-pressure engine; and
The temperature required in the first decompression step is a temperature higher than the temperature required by the high-pressure engine, the method for re-liquefying the boil-off gas of the ship. The method of claim 8,
The total cooling load in the temperature control step is a fixed value,
A load allocation step of distributing a cooling load to be allocated to the first cooling step and a cooling load to be allocated to the second cooling step among the total cooling loads of the temperature control step to each step; further comprising, boil-off gas of the ship Reliquefaction method. The method according to any one of claims 7 to 10,
A second decompression step of reducing the boil-off gas cooled in the cooling step to an allowable pressure of the liquefied gas storage tank; further comprising, reliquefaction of boil-off gas of a ship. The method of claim 11,
By gas-liquid separating the boil-off gas depressurized in the second decompression step, the re-liquefied boil-off gas in a liquid state is supplied to the liquefied gas storage tank, and the boil-off gas in the gaseous state is transferred to the pre-compression boil-off gas flow to the cooling step. Gas-liquid separation step of confluence; further comprising, the boil-off gas reliquefaction method of the ship.

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