KR101767560B1 - Reliquefaction System And Method For Boil Off Gas - Google Patents

Abstract

A vapor gas remelting system and method are disclosed. The evaporation gas re-liquefaction system of the present invention comprises a compressor for compressing evaporative gas generated in a storage tank provided in a ship or an offshore structure; A first heat exchanger in which the evaporated gas compressed in the compressor is heat-exchanged with the evaporated gas to be introduced into the compressor; And a first expansion means for expanding the compressed evaporated gas heat-exchanged in the first heat exchanger, wherein the compressor is supplied with lubricating oil for preventing wear and cooled through a part of the first heat exchanger, And an oil separator for separating the lubricating oil contained in the compressed evaporated gas and reintroducing the lubricating oil into the first heat exchanger.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

More particularly, the present invention relates to a system and a method for re-liquefying an evaporation gas, and more particularly, to a system and method for re-liquefying an evaporation gas, The present invention relates to an evaporative gas re-liquefaction system capable of re-liquefying an evaporated gas by thermal expansion, and an oil separator provided to prevent lubricating oil contained in compressed evaporative gas from being removed, thereby preventing clogging of a pipe of the heat exchanger.

Liquefied natural gas (hereinafter referred to as "LNG") is a colorless transparent liquid obtained by cooling methane-based natural gas to about -162 ° C. and liquefying it. / 600. ≪ / RTI > Therefore, it is very efficient to transport liquefied LNG when transporting natural gas. For example, an LNG carrier that can transport (transport) LNG is used.

Since the liquefaction temperature of natural gas is a cryogenic temperature of -163 ° C at normal pressure, LNG is easily evaporated even if its temperature is slightly higher than the normal pressure of -163 ° C. LNG storage tanks of LNG carriers are heat-treated, but since external heat is continuously transferred to LNG storage tanks, LNG is constantly spontaneously vaporized in LNG storage tanks during LNG transportation by LNG carrier, Boil-off gas (BOG) is generated in the storage tank.

BOG is a kind of LNG loss, which is an important problem in the transport efficiency of LNG. When the evaporation gas accumulates in the LNG storage tank, the pressure in the LNG storage tank is excessively increased, Have been studied.

Recently, for the treatment of BOG, BOG is re-liquefied and returned to the storage tank, and BOG is used as energy source of engine of ship. In addition, the surplus BOG is combusted in a gas combustion unit (GCU).

The gas combustion unit burns surplus BOG inevitably for controlling the pressure of the storage tank when the BOG can not be utilized otherwise, resulting in a waste of the chemical energy possessed by the BOG by combustion.

When a dual fuel (DF) engine is applied as the main propulsion unit in the propulsion system of the LNG carriers, the evaporative gas generated in the LNG storage tank can be used as the fuel of the DF engine to process the evaporative gas, If the amount of evaporative gas generated in the LNG storage tank exceeds the amount of fuel used in the propulsion of the ship in the DF engine, the evaporation gas may be sent to a gas burner to incinerate it to protect the LNG storage tank.

Application No. 10-2010-0116987

Cryogenic LNG is very sensitive to changes in the external environment such as temperature, and since it is continuously vaporized in the cargo hold during the operation of the ship, a considerable amount of BOG (boil off gas, evaporation gas) is generated. As the BOG becomes excessive in the storage container, the pressure in the container rises and the container can not withstand the internal pressure and there is a danger of explosion. Therefore, the BOG is discharged and liquefied and then stored or burned and removed do. The amount of evaporative gas (BOG) generated in the storage vessel is about 0.05 vol% / day even when the vessel is equipped with a heat insulating structure. The conventional liquefied natural gas carrier carries 4 to 6 tons / It is known that about 300 tons of liquefied natural gas is vaporized and gasified in a single operation.

In order to re-liquefy the evaporation gas, the evaporation gas in the storage tank is discharged to the outside of the storage tank and re-liquefied through the re-liquefaction device including the refrigeration cycle. At this time, the evaporation gas is cooled by the ultra- Nitrogen, mixed refrigerant, and the like, and then returned to the storage tank. In such a re-liquefying apparatus through a refrigeration cycle, the entire system control is complicated due to the complexity of operation, and there is a problem that a lot of power is consumed.

In order to liquefy such a large amount of BOG, a complicated re-liquefying device and a large amount of energy are required. In the case of burning and removing the fuel, the fuel can not be used and is discarded. A system is required.

SUMMARY OF THE INVENTION The present invention has been made in order to solve such a problem, and it is an object of the present invention to provide a system capable of efficiently re-liquefying evaporated gas generated in a cargo hold of a ship.

According to an aspect of the present invention, there is provided a compressor for compressing evaporative gas generated in a storage tank provided in a ship or an offshore structure;

A first heat exchanger in which the evaporated gas compressed in the compressor is heat-exchanged with the evaporated gas to be introduced into the compressor; And

A first expansion means for expanding the compressed evaporated gas heat-exchanged in the first heat exchanger,

The compressor is supplied with lubricating oil for preventing wear,

Further comprising an oil separator for separating and reintroducing the lubricant contained in the compressed evaporated gas, which is cooled through a portion of the first heat exchanger, into the first heat exchanger.

Preferably, the compressed evaporated gas is cooled in the first heat exchanger to the condensation temperature of the lubricating oil and introduced into the oil separator.

Preferably, the compressor is a multi-stage compressor in which compression and intermediate cooling are repeated, and the compressed evaporated gas may be introduced into the heat exchanger through at least three stages of compression in the multi-stage compressor.

Preferably, the gas-liquid separator for gas-liquid separating the vaporized gas that has been thermally expanded through the first expansion means, second expansion means for expanding the gas separated from the gas-liquid separator, The second heat exchanger may further include a second heat exchanger for further cooling the compressed evaporated gas heat-exchanged in the first heat exchanger by heat exchange with a gas passing through the second expansion means and supplying the cooled evaporated gas to the first expansion means.

Preferably, the gas passing through the second heat exchanger is introduced into the first heat exchanger together with the evaporated gas generated in the storage tank, and the liquid separated from the gas-liquid separator can be recovered to the storage tank.

Advantageously, said first expansion means may comprise a line-Thomson expansion valve and an expander.

Preferably, the apparatus further comprises a main engine provided in the ship or an offshore structure and supplied with the evaporated gas compressed by the compressor, and a sub engine supplied with the evaporated gas compressed through at least a part of the compressor, Can be supplied with the evaporated gas compressed to 150 to 400 bar by the compressor.

According to another aspect of the present invention, there is provided a method of compressing an evaporative gas, comprising: 1) compressing an evaporative gas generated in a storage tank of a ship or an offshore structure into a compressor;

2) introducing the compressed evaporated gas into a heat exchanger to heat-exchange the evaporated gas before compression generated in the storage tank; And

3) subjecting the heat-exchanged evaporated gas to thermal expansion and gas-liquid separation,

Wherein the compressor is supplied with lubricating oil for preventing wear and the evaporated gas partially cooled in the heat exchanger is reintroduced into the heat exchanger after separating the lubricating oil contained in the evaporated gas. do.

Preferably, the gas separated by the gas-liquid separation is thermally expanded, and the evaporation gas can be further cooled by heat exchange with the evaporation gas prior to the step 3).

The evaporation gas re-liquefaction system of the present invention compresses the evaporation gas generated in the storage tank, heat-exchanges the evaporation gas with the evaporation gas to be introduced into the compressor, It is possible to provide an oil separator capable of removing the lubricating oil to prevent the lubricating oil contained in the compressed evaporation gas.

It is possible to prevent clogging of the heat exchanger such as PCHE which is vulnerable to foreign substances through the removal of the lubricating oil, thereby improving the operational stability of the re-liquefaction system and reducing maintenance and equipment replacement costs. Further, the present invention is a system which can re-liquefy evaporated gas by using the cold heat of the evaporated gas itself generated in the storage tank, and does not require a separate refrigerant system, so that the initial installation cost and equipment size can be reduced, Maintenance is also convenient.

In addition, by not providing a re-liquefaction device consuming a large amount of energy for re-liquefaction, it is possible to reduce the driving cost of the device for re-liquefaction and reduce the amount of natural gas wasted by combustion etc. through effective re- .

Figure 1 schematically depicts an evaporative gas re-liquefaction system in accordance with one embodiment of the present invention.
2 schematically shows a vaporization gas remelting system according to another embodiment of the present invention.
FIG. 3 schematically illustrates a vaporization gas remelting system according to another embodiment of the present invention.

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

Hereinafter, the present invention will be described in detail with reference to the preferred embodiments of the present invention with reference to the accompanying drawings. Like reference symbols in the drawings denote like elements.

FIG. 1 is a schematic block diagram of an evaporative gas treatment system in a marine structure according to an embodiment of the present invention.

1 shows an example in which the evaporative gas treatment system of the present invention is applied to a high-pressure natural gas injection engine capable of using natural gas as fuel, that is, an LNG carrier equipped with an ME-GI engine. However, The systems for treatment can be applied to all types of offshore structures equipped with liquefied gas storage tanks, namely LNG carriers, vessels such as LNG RV, marine plants such as LNG FPSO, LNG FSRU.

1, the evaporative gas (NBOG) generated and discharged from the storage tank (11) storing the liquefied gas is supplied to the evaporation gas processing system of the offshore structure according to the first embodiment of the present invention, Is fed along the feed line L1, compressed in the evaporative gas compression section 13, and then supplied to the high-pressure natural gas injection engine, for example, the ME-GI engine. The evaporation gas is compressed to a high pressure of about 150 to 300 bara by the evaporation gas compression unit 13 and then supplied as fuel to a high pressure natural gas injection engine such as an ME-GI engine.

The storage tank has a sealing and thermal barrier to store liquefied gases such as LNG in cryogenic conditions, but it can not completely block the heat transmitted from the outside. Accordingly, the evaporation of the liquefied gas is continuously performed in the storage tank 11, and the evaporation gas in the storage tank 11 is discharged through the evaporation gas discharge line L1 to maintain the pressure of the evaporation gas at an appropriate level .

A discharge pump (12) is installed in the storage tank (11) to discharge the LNG to the outside of the storage tank, if necessary.

The evaporative gas compression section 13 may include at least one evaporative gas compressor 14 and at least one intermediate cooler 15 for cooling the evaporated gas whose temperature has increased while being compressed in the evaporative gas compressor 14. In Fig. 1, there is illustrated a multi-stage compressed evaporative gas compression section 13 including five evaporative gas compressors 14 and five intermediate coolers 15. The evaporation gas compression section 13 can be configured, for example, to compress the evaporation gas to about 301 bara.

The compressed gas is supplied to the high-pressure natural gas injection engine through an evaporation gas supply line (L1). The compressed gas is supplied to the high-pressure natural gas injection engine To the high-pressure natural gas injection engine. According to the present invention, when the evaporated gas discharged from the storage tank 11 and compressed by the evaporated gas compression unit 13 is referred to as a first stream, the first stream of the compressed evaporated gas is divided into the second stream and the third stream Stream, the second stream is supplied as fuel to the high pressure natural gas injection engine, and the third stream is liquefied and returned to the storage tank.

At this time, the second strip is supplied to the high-pressure natural gas injection engine through the evaporation gas supply line (L1), and the third stream is returned to the storage tank (11) through the evaporation gas return line (L3). A heat exchanger (21) is installed in the evaporation gas return line (L3) so that the third stream of the compressed evaporation gas can be liquefied. In the heat exchanger (21), the third stream of the compressed evaporated gas is discharged from the storage tank (11), and then heat-exchanged with the evaporated gas supplied to the evaporated gas compression unit (13).

Because the flow rate of the first stream of evaporated gas before being compressed is greater than the flow rate of the third stream, the third stream of compressed evaporated gas may be liquefied by receiving cold heat from the first stream of evaporated gas before being compressed. As described above, in the heat exchanger 21, the extremely low-temperature evaporation gas immediately after being discharged from the storage tank 11 is exchanged with the high-pressure evaporation gas compressed by the evaporation gas compression unit 13 to liquefy the evaporation gas at the high pressure state .

The evaporated gas LBOG liquefied in the heat exchanger 21 is reduced in pressure while passing through the expansion valve 22 and supplied to the gas-liquid separator 23 in a vapor-liquid mixed state. The LBOG can be decompressed to approximately atmospheric pressure while passing through the expansion valve 22. The liquefied evaporated gas is separated from the gas and liquid components in the gas-liquid separator 23, and the liquid component, that is, the LNG is transferred to the storage tank 11 through the evaporated gas return line L3, and the gas component, And is merged into the evaporation gas discharged from the storage tank 11 through the evaporation gas recycle line L5 and supplied to the evaporation gas compression unit 13. More specifically, the evaporation gas recycle line L5 extends from the upper end of the gas-liquid separator 23 and is connected to the evaporation gas supply line L1 on the upstream side of the heat exchanger 21.

In the above description, the heat exchanger 21 is provided in the evaporation gas return line L3, but in the heat exchanger 21, the first stream of the evaporation gas being fed through the evaporation gas supply line L1 The heat exchanger 21 is installed in the evaporation gas supply line L1 since heat exchange is performed between the third stream of the evaporation gas being transferred through the evaporation gas return line L3.

The evaporation gas recirculation line L5 may be further provided with another expansion valve 24 so that the gas component discharged from the gas-liquid separator 23 can be decompressed while passing through the expansion valve 24. The third stream of the evaporated gas supplied to the gas-liquid separator 23 after being liquefied in the heat exchanger 21 is heat-exchanged with the gas component separated by the gas-liquid separator 23 and conveyed through the evaporation gas recycle line L5, The evaporator gas recycle line (L5) is equipped with a cooler (25) to further cool the stream. That is, in the cooler 25, the evaporation gas in the high-pressure liquid state is further cooled by heat exchange with the natural gas in the low-pressure cryogenic gaseous state.

Although the cooler 25 has been described as being installed in the evaporative gas recirculation line L5 for convenience of explanation, in the cooler 25 in reality, the third stream of the evaporative gas being fed through the evaporative gas return line L3, The cooler 25 is provided in the evaporation gas return line L3 since heat exchange is performed between the gas components being transferred through the gas recirculation line L5.

If the amount of evaporative gas generated in the storage tank 11 is higher than the amount of fuel required by the high-pressure natural gas injection engine and excess evaporative gas is expected to be generated, the compressed or stepped Is branched through the evaporation gas branch lines (L7, L8) and used in the evaporation gas consumption means. GCU, DF Generator, Gas Turbine, DFDE, etc., which can use natural gas relatively low pressure compared to ME-GI engine, can be used as evaporative gas consumption means.

FIG. 2 schematically shows a re-liquefaction system for a vaporized gas according to another embodiment of the present invention.

2, the evaporation gas re-liquefaction system of the present embodiment includes a compressor 100 for compressing the evaporation gas generated in a storage tank T provided in a ship or an offshore structure, A first heat exchanger (200) for heat exchange with the evaporation gas to be introduced into the compressor, and a first expansion means (400a) for thermally expanding the compressed evaporation gas heat exchanged in the heat exchanger.

The compressor 100 of the present embodiment is a multi-stage compressor in which compression and intermediate cooling are repeated like the evaporative gas compression unit (13 in Fig. 1) shown in Fig. 1 of the above embodiment, The evaporation gas can be compressed to 150 to 400 bar.

At this time, the compressor 100 of the present embodiment is supplied with oil and lubricating oil for preventing wear of the piston, and an oil separator 300 for removing the oil contained in the compressed evaporative gas introduced into the heat exchanger from the compressor is provided at the downstream side of the compressor do.

The compressed evaporative gas introduced into the first heat exchanger 200 from the compressor 100 is introduced into the heat exchanger through at least three stages, preferably four to five stages of compression in the multi-stage compressor, Lubricating oil, which is supplied to the compressor 100, may be mixed.

As such a compressor 100, for example, a compressor having three cylinders at the front end operating in an oil-free manner and two cylinders at the rear end operating in an oil-lubricated manner, Can be used. This is because there is a great risk that the piston ring of the cylinder will wear down toward the rear end, so that lubrication oil is supplied to the cylinder at the rear end to prevent wear of the piston ring. Thus, when using such a compressor, the compressed evaporated gas may include lubricating oil when it is branched in four or more stages. Accordingly, the lubricating oil contained in the compressed evaporated gas can be introduced into the heat exchanger 200.

At this time, for example, a PCHE (Printed Circuit Heat Exchanger) may be applied to the heat exchanger 200 in this embodiment.

In the PCHE heat exchanger, the fluid to be heat-exchanged flows into the heat exchanger from different directions and passes through the heat exchanger. Such a PCHE heat exchanger has a very wide heat exchangeable temperature range of about -200 to 900 ° C. and a large heat transfer area per unit volume of the heat exchanger, exhibits a high heat transfer rate and can be applied to various fluids such as gas and liquid and two- There are advantages to be able to use.

On the other hand, since the circuit inside the heat exchanger is very small, there may be a problem that the pipeline is clogged when foreign matter is introduced. Therefore, the oil for preventing compressor wear, that is, the lubricating oil contained in the compressed evaporated gas, may cause pipe clogging of the PCHE heat exchanger, which may cause the operation of the re-liquefaction system to be interrupted.

Particularly, since the evaporated gas compressed through the heat exchange in the first heat exchanger 200 is cooled, the viscosity of the lubricating oil contained in the evaporated gas also increases due to cooling. As a result, the risk of clogging in the heat exchanger such as the PCHE heat exchanger Big. In this embodiment, an oil separator 300 is provided so as to remove oil from the compressed evaporative gas in the compressor 100 in order to solve this problem.

Thus, the oil separator 300 is provided to remove the oil contained in the compressed evaporated gas, thereby preventing clogging of the pipe in the heat exchanger. The oil separator 300 may be a known means capable of separating droplets from the gas, which separates the oil mixed in the compressed evaporated gas.

Particularly, in order to separate the lubricating oil smoothly, this embodiment separates the partially evaporated gas through a part of the first heat exchanger 200 and introduces it into the oil separator 300. In the first heat exchanger 200, And the evaporator gas is supplied to the oil separator 300 in the section cooled to a temperature of preferably -10 to 0 占 폚. The evaporated gas from which lubricating oil has been removed via the oil separator 300 is reintroduced into the first heat exchanger 200.

On the other hand, the temperature of the evaporation gas increases as it is compressed. This is stored in the evaporation gas, which is generated in the storage tank T and is heat-exchanged in the first heat exchanger 200 by the evaporation gas to be introduced into the compressor 100 And conveys the cold heat of the evaporated gas supplied from the tank.

Although a system having one heat exchanger 200 is shown in FIG. 2 of the present embodiment, a plurality of heat exchangers may be provided so that the remaining heat exchangers are installed in a part of the heat exchanger even when maintenance is performed such as clogging of pipes, Heat exchange of the evaporation gas may be performed through the evaporator, so that the system can be configured to re-liquefy the evaporation gas without interruption.

In the present embodiment, the evaporated gas passing through the heat exchanging portion is introduced into the first expansion means, for example, the expansion valve 400a along the flow path, and the evaporated gas that has passed through the first expansion means 400a and is thermally expanded is introduced into the gas- ) To be subjected to gas-liquid separation. The expansion means may be provided in plural.

The separated natural gas is combined with the flow of the evaporative gas generated in the storage tank T and is supplied to the first heat exchanger 200, Lt; / RTI >

At this time, in this embodiment, the gas separated from the gas-liquid separator 500 is subjected to heat exchange between the second expansion means 600 cooled by the single thermal expansion and the gas passing through the second expansion means 600, And a second heat exchanger (700) for further cooling the evaporated gas before being supplied to the first expansion means (400) after passing through the first expansion means (200).

The gas separated from the gas-liquid separator 500 through the second expansion means 600 and heat-exchanged with the evaporation gas in the second heat exchanger is joined to the flow of the evaporation gas generated in the storage tank, and is supplied to the first heat exchanger 200 .

The first and second expansion means 400 and 600 may be used in the same manner as in the present embodiment shown in Figure 2 and may include an expander, 400b may be used. The cooled evaporated gas is mono-expanded through a decompression device such as a pressure reducing valve, and the pressure is lowered.

The evaporated gas generated in the storage tank T is re-liquefied under compression, cooling, and thermal expansion processes, and the liquefied natural gas is separated through the gas-liquid separator 500 and recovered into the storage tank T.

In this way, the evaporation gas is compressed by the compressor 100 and re-liquefied and recovered by using the low temperature of the evaporation gas supplied from the storage tank T without a separate refrigerant system, thereby preventing the evaporation gas from being wasted by combustion or the like in the GCU. In addition, when the gas is compressed by the compressor 100, the power consumption of the compressor 100 is kept constant until a certain amount of the gas flow rate, and then the power consumption is increased. However, If compressed and sent to fuel or liquefied, the evaporative gas can be effectively treated without additional power consumption.

Liquid separator 500 may further include a pressure reducing valve for further reducing the pressure of the liquefied natural gas recovered in the storage tank.

Meanwhile, the evaporated gas compressed by the compressor 100 at a pressure of 150 to 400 bar can be supplied to the main engine E1 fuel of the ship or an offshore structure. In particular, the engine at this time is a high pressure gas injection Engine, preferably an ME-GI engine. In addition to the ME-GI engine, a sub-engine E2 such as a DFDE or DF generator, which is supplied with fuel at a pressure lower than that of the ME-GI engine, is additionally provided to branch the compressed gas in the middle of the compressor They can also be supplied as fuel.

However, the compressor 100 for compressing the evaporation gas (BOG) to a high pressure of 150 to 400 bara (absolute pressure) required by the MEGI engine described above is considerably expensive, bulky and consumes a large amount of power. For example, 2 MW of power is consumed to fuel the ME-GI engine by driving a 5-stage compressor composed of multi-stages, while 600 kW is required when only the second stage is used and the remaining three stages are idling.

Accordingly, when the evaporator is re-liquefied through only a part of the multi-stage compressor without compressing the evaporation gas to 150 to 400 bara, the consumption power of the compressor 100 can be greatly reduced. In this case, the main engine El, such as the MEGI engine, can configure the system to receive the liquefied natural gas stored in the storage tank via a pump (not shown) and a vaporizer (not shown) instead of the evaporated gas.

Thus, when the amount of LNG stored in the storage tank is small and the amount of BOG generated is smaller than the required amount of fuel in the MEGI engine, such as a ballast condition, the evaporative gas is supplied through the fuel supply of the sub-engine E2 and the re-liquefaction of the evaporative gas, ) Is advantageous in terms of energy efficiency to supply LNG as fuel through a pump. Since the evaporation gas and the LNG have different methane values, it is possible to stably manage the load of the engine by supplying the fuel to only one of the main engine and the sub-engine, Do.

As described above, in this embodiment, 1) compressing evaporative gas generated from a storage tank of a ship or an offshore structure by a compressor; 2) introducing the compressed evaporated gas into a heat exchanger to heat-exchange the evaporated gas before compression generated in the storage tank; And 3) subjecting the heat exchanged evaporated gas to a thermal expansion and gas-liquid separation, wherein the compressor is supplied with lubricating oil for preventing wear, separates the lubricating oil contained in the partially cooled evaporated gas in the heat exchanger, It is possible to prevent clogging of the pipe of the heat exchanger.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Accordingly, such modifications or variations are intended to fall within the scope of the appended claims.

100: compressor 200: first heat exchanger
300: Oil separator 400: First expansion means
500: gas-liquid separator 600: second expansion means
700: second heat exchanger T: storage tank
E1: Main engine E2: Secondary engine

Claims (10)

A system for re-liquefying evaporative gas generated in a storage tank mounted on a ship or an offshore structure,
A compressor for compressing the evaporated gas discharged from the storage tank and supplied with oil for abrasion prevention;
A first heat exchanger in which the evaporated gas compressed by the compressor is heat-exchanged with the evaporated gas to be supplied to the compressor;
Oil separating means for separating the lubricating oil contained in the compressed evaporative gas cooled through the part of the first heat exchanger and supplying the lubricating oil to the first heat exchanger again; And
And a first expansion means for expanding the compressed evaporated gas heat-exchanged in the first heat exchanger,
Wherein the evaporated gas compressed by the compressor is cooled to a condensing temperature of the lubricating oil through a part of the first heat exchanger and then sent to the oil separating means, Fed to the heat exchanger, cooled to a temperature required for re-liquefaction, and then sent to the first expansion means. The method according to claim 1,
Wherein the compressor is a multi-stage compressor in which compression and intermediate cooling are repeated, and the compressed evaporated gas is sent to the heat exchanger through at least three stages of compression in the multi-stage compressor. The method according to claim 1,
Further comprising a gas-liquid separator for gas-liquid separating the evaporated gas expanded by said first expansion means,
And the liquid separated by the gas-liquid separator is recovered to the storage tank. The method of claim 2,
Wherein the evaporated gas branched after being compressed by four or more stages of the compressor is sent to the first heat exchanger and passed through the oil separating means. The method of claim 2,
Wherein the two cylinders at the rear end of the compressor operate in an oil feed lubrication manner. The method according to any one of claims 1 to 5,
Wherein the first expansion means is an expansion valve or an expander. The method according to any one of claims 1 to 5,
A main engine which is supplied with the evaporated gas compressed by the compressor; And
Further comprising: a sub-engine which is supplied with the compressed evaporated gas through at least a part of the compressor,
Wherein the main engine is supplied with an evaporated gas compressed to 150 to 400 bar by the compressor. 1) compressing the evaporated gas discharged from the storage tank by a compressor;
2) cooling the evaporated gas compressed in the step 1) by heat exchange with the evaporated gas before compression discharged from the storage tank; And
3) expanding the evaporated gas heat-exchanged in the step 2)
The compressor is supplied with lubricating oil for preventing wear, and the fluid which has been subjected to only a part of the cooling process in the step 2) is subjected to the remaining cooling process after the lubricating oil is separated,
The fluid having passed through only a part of the cooling process in the step 2) is cooled to the condensation temperature of the lubricating oil, the lubricating oil is separated, the separated fluid is cooled to the temperature required for the re-liquefaction after the remaining cooling process, Of the evaporation gas. The method of claim 8,
4) separating the fluid that has been partially or fully re-liquefied in the step 3)
And the liquid separated in the step (4) is recovered to the storage tank. The method according to any one of claims 1 to 5,
Wherein the oil separating means is an oil separator.

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