BOIL-OFF GAS RE-LIQUEFYING SYSTEM AND SHIP COMPRISING SAME

The present invention relates to a boil-off gas re-liquefying system and a ship including the same, the system, as a system for processing a liquefied gas, which is a heavy hydrocarbon, including: a compressor compressing a boil-off gas generated from a liquefied gas storage tank in multiple stages; a condenser condensing the boil-off gas compressed in the compressor; an intercooler mutually heat-exchanging between a part of the liquid-phase boil-off gas condensed in the condenser and the rest, transferring a gas-phase boil-off gas generated by heat exchange to the compressor, and transferring the liquid-phase boil-off gas to the liquefied gas storage tank; and a liquefied gas pump pressurizing the liquefied gas of the liquefied gas storage tank, wherein the liquefied gas pump transfers a liquefied gas to the intercooler to liquefy the gas-phase boil-off gas in the intercooler.

TECHNICAL FIELD

The present invention relates to a boil-off gas re-liquefying system and a ship including the same.

BACKGROUND ART

Among ships that sail the sea with various types of loaded cargo, liquefied gas carriers that transport liquefied gas such as liquefied natural gas or liquefied petroleum gas are provided with a storage tank that forcibly liquefies a gas with a boiling point lower than room temperature and stores it in a liquid state.

Liquefied natural gas is produced by cooling and liquefying methane (CH4) obtained by refining natural gas collected from gas fields, and it is a colorless and transparent liquid that contains almost no pollutants and has a high calorific value, and thus is an excellent fuel. On the other hand, liquefied petroleum gas is a liquid made of a gas including propane (C3H8) and butane (C4H10) that come out from oil fields together with petroleum as the main ingredients, and is widely used as a fuel for household, business, industrial, and automobile purposes. Liquefied natural gas is reduced to 1/600 in volume by liquefaction, and liquefied petroleum gas is reduced to 1/260 in volume as propane and to 1/230 in volume as butane by liquefaction, and so they have high storage efficiency.

However, although the storage tank storing the liquefied gas has an insulation function, it cannot completely block the vaporization of the liquefied gas. Therefore, in the storage tank, boil-off gas in a gaseous state is generated by the evaporation of the liquefied gas, and since the boil-off gas increases the internal pressure of the storage tank, it needs to be discharged from the storage tank for safety.

To lower the internal pressure of the storage tank, the boil-off gas discharged from the storage tank is combusted and discarded through a gas combustion unit. However, since the boil-off gas is also a part of the cargo carried by a ship, the emission of the boil-off gas is a problem because it reduces the reliability of cargo transportation.

Therefore, in recent years, continuous research and development have been carried out on methods for effectively treating the boil-off gas generated from a storage tank without discarding it.

DISCLOSURE

Technical Problem

The present invention was created to solve the problems of the prior art as described above, and an object of the present invention is to provide a boil-off gas re-liquefying system and a ship including the same, wherein the boil-off gas re-liquefying system may increase re-liquefaction efficiency by suppressing the generation itself of a non-condensable gas that may not be condensed upon re-liquefaction of a liquefied gas by using the liquefied gas or by separating the non-condensable gas and processing it.

Technical Solution

A boil-off gas re-liquefying system according to one aspect of the present invention, as a system for processing a liquefied gas, which is a heavy hydrocarbon, comprises: a compressor compressing a boil-off gas generated from a liquefied gas storage tank in multiple stages; a condenser condensing the boil-off gas compressed in the compressor; an intercooler mutually heat-exchanging between a part of the liquid-phase boil-off gas condensed in the condenser and the rest, transferring a gas-phase boil-off gas generated by heat exchange to the compressor, and transferring the liquid-phase boil-off gas to the liquefied gas storage tank; and a liquefied gas pump pressurizing the liquefied gas of the liquefied gas storage tank, wherein the liquefied gas pump transfers a liquefied gas to the intercooler to liquefy the gas-phase boil-off gas in the intercooler.

Specifically, the intercooler may depressurize a part of the liquid-phase boil-off gas condensed in the condenser with a depressurizing valve and then storing in the inside and pass the rest through the inside to mutually heat exchange with the boil-off gas, and the liquefied gas pump may inject the liquefied gas to the inside of the intercooler so that the liquefied gas drops the temperature of a part of the boil-off gas stored in the intercooler and cools the rest of the boil-off gas passing through the inside of the intercooler.

Specifically, the liquefied gas may be a mixture of a first substance and a second substance with different boiling points, and the intercooler may transfer the first material with a relatively low boiling point to the compressor as a gas-phase boil-off gas during heat exchange with a boil-off gas.

Specifically, the liquefied gas pump may transfer the liquefied gas to the intercooler to limit the evaporation amount of the first material in the intercooler within a preset value.

Specifically, as the system operation time elapses, the first material may continue to circulate through the compressor, the condenser, and the intercooler so that the proportion of the first material in the boil-off gas flowing through the condenser increases, and the liquefied gas pump may transfer the liquefied gas to the intercooler and reduce the flow rate of the first substance transferred from the intercooler to the compressor so that the proportion of the first material in the boil-off gas flowing through the condenser is within a preset value.

Specifically, the liquefied gas pump transfers the liquefied gas to the intercooler when the proportion of the first substance in the boil-off gas flowing through the condenser is greater than or equal to a preset value.

Specifically, a ship according to one aspect of the present invention has the boil-off gas re-liquefying system.

Advantageous Effects

A boil-off gas re-liquefying system and a ship including the same according to the present invention can innovatively improve re-liquefaction performance by preventing a non-condensable gas from being generated using a low-temperature liquefied gas in a re-liquefaction process of a liquefied petroleum gas or by separating a non-condensable gas and cooling and liquefying it.

MODES OF THE INVENTION

The objects, specific advantages, and novel features of the present invention will become more apparent from the following detailed description and preferred embodiments taken in conjunction with the accompanying drawings. In the present specification, when adding reference numerals to components in each drawing, it should be noted that the same components are given the same numbers as much as possible even when they are shown in different drawings. In addition, in describing the present invention, when it is considered that a detailed description of related known technology or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted.

In the present specification, a liquefied gas is a heavy hydrocarbon, which may be liquefied petroleum gas (LPG; propane, butane, etc.), but it is not limited thereto, and it may encompass any substance (propylene, ammonia, hydrogen, etc.) that is forcibly liquefied for storage because the boiling point is lower than room temperature and that has a calorific value.

In addition, it is noted that in the present specification, a liquefied gas/boil-off gas is classified based on the state inside a tank, and is not necessarily limited to a liquid phase or a gas phase due to the name.

The present invention includes a ship provided with a boil-off gas re-liquefying system described below. At this time, it is noted that a ship is a concept that includes all of gas carriers, merchant ships that transport non-gas cargo or people, floating storage regasification unit (FSRU), floating production storage and offloading (FPSO), bunkering vessels, offshore plants, etc., but it may be a liquefied petroleum gas carrier as an example.

Although not shown in the drawings of the present invention, a pressure sensor (PT), a temperature sensor (TT), or the like may, of course, be provided at an appropriate position without limitation, and values measured by each sensor are used in the operation of the configurations described below in a variety of ways without limitation.

FIG.1shows a conceptual diagram of a boil-off gas re-liquefying system according to a first embodiment of the present invention. Referring toFIG.1, a boil-off gas re-liquefying system1according to one embodiment of the present invention includes a liquefied gas storage tank10, a buffer20, a compressor30, a condenser40, a receiver50, an intercooler60, a pressure control valve70, a liquefied gas pump90, and a fuel supply portion100.

A liquefied gas storage tank10stores a liquefied gas such as liquefied petroleum gas or ammonia. One or more liquefied gas storage tanks10may be provided inside or outside a ship, and may liquefy a gas with a boiling point lower than room temperature and store it in a cryogenic state.

A liquefied gas storage tank10may be of a membrane type, an independent type, or a pressure vessel type, but it is not particularly limited. However, regardless of the type, a part of a liquefied gas is spontaneously vaporized inside a liquefied gas storage tank10to generate a boil-off gas, and the boil-off gas may be problematic because it causes an increase in the internal pressure of a liquefied gas storage tank10. Therefore, in the present embodiment, a boil-off gas is discharged to the outside of a liquefied gas storage tank10, and the discharged boil-off gas may be re-liquefied and returned to the liquefied gas storage tank10.

Alternatively, in the present invention, a boil-off gas may be used as a fuel for a demand site (reference numeral not shown), and at this time, the demand site may be an engine, a turbine, a boiler, a fuel cell, a burner, etc. provided on a ship, and it may be a propulsion engine that propels a ship or a power generation engine to cover the internal power load of a ship.

A liquefied gas storage tank10may be provided with a boil-off gas discharge line L10for discharging a boil-off gas, and the boil-off gas discharge line L10may extend from a liquefied gas storage tank10to be connected to a boil-off gas re-liquefying system1.

A buffer20is connected to a boil-off gas discharge line L10and temporarily stores a boil-off gas discharged from a liquefied gas storage tank10. A buffer20is a separator separating a gas phase and a liquid phase, and it performs gas-liquid separation of a boil-off gas discharged from a liquefied gas storage tank10and supplies only a boil-off gas of a gaseous state, thereby preventing damage to a compressor30.

A gas-phase boil-off gas separated in a buffer20may be transferred to a compressor30through a boil-off gas liquefaction line L20. A boil-off gas liquefaction line L20is a component that extends from a buffer20and transfers a boil-off gas to a liquefied gas storage tank10via a condenser40, and a boil-off gas liquefaction line L20may be provided with a compressor30, a condenser40, a receiver50, a pressure control valve70, etc. In addition, a boil-off gas liquefaction line L20may be provided to pass through an intercooler60.

A compressor30compresses a boil-off gas generated from a liquefied gas storage tank10. A compressor30may be of a centrifugal or reciprocating type, and may be provided in multiple stages including a plurality of compression stages. In addition, a compressors30may be provided in parallel for backup or load sharing.

A compressor30may compress a boil-off gas flowing in at around 1 bar to 10 to 100 bar, and when a boil-off gas is compressed by a compressor30, the boiling point of the boil-off gas increases. Therefore, a compressed boil-off gas may be in a liquefiable state even when it is not cooled to the boiling point at atmospheric pressure (for example, −55 degrees in the case of LPG).

A compressor30may be composed of three stages, and may compress a boil-off gas to approximately 4 bar in a first stage30a, approximately 10 bar in a second stage30b, and approximately 20 to 30 bar in a third stage30c. Of course, the pressure of a boil-off gas compressed by the compressor30and compression stages is not particularly limited.

A plurality of compression stages may be provided in series in a boil-off gas liquefaction line L20connected from a buffer20to a condenser40to form a multi-stage compressor30. In an intermediate stage between compression stages on a boil-off gas liquefaction line L20, a first intercooler60aand a second intercooler60bmay be connected as an intercooler60.

A low-pressure boil-off gas that has escaped from a compressor first stage30apasses through a second intercooler60band is then transferred to a compressor second stage30b, and a medium-pressure boil-off gas that has escaped from the compressor second stage (30b) passes through a first intercooler60aand then is transferred to a compressor third stage30c, and it escapes from the compressor third stage30cas a high-pressure boil-off gas and is transferred to a condenser40.

At this time, as will be described later, the intercooler60is a cooling facility that uses a depressurized boil-off gas as a refrigerant without a separate refrigerant, and it is capable of cooling a low-pressure boil-off gas or a medium-pressure boil-off gas flowing in from a compressor30. Therefore, an intercooler60may implement cooling at an intermediate stage of the compressor30.

Of course, a compressor30may allow a boil-off gas to be transferred between a first stage30aand a second stage30band between a second stage30band a third stage30c, bypassing an intercooler60, a bypass may be controlled in various ways depending on variables such as the internal pressure of an intercooler60and the temperature of a boil-off gas.

A boil-off gas is discharged from a liquefied gas storage tank10at around-50 degrees, and after passing through a buffer20, the discharged boil-off gas may flow into a compressor first stage30aat around 1 bar and −20 degrees.

After that, the boil-off gas is discharged from the compressor first stage30aat about 4 bar and about 40 degrees and flows into a second intercooler60b, and after being cooled to about 30 degrees within the second intercooler60b, the boil-off gas is transferred to a compressor second stage30b.

After that, the boil-off gas is discharged from the compressor second stage30bin a state of about 10 bar and about 70 degrees and flows into a first intercooler60a, and after being cooled to about 60 degrees in the first intercooler60a, it is transferred to a compressor third stage30c. Finally, the boil-off gas is discharged from the compressor third stage30cin a state of around 20 to 30 bar and around 100 degrees, and then it may be cooled to around 40 degrees in a condenser40.

However, in situations such as when the temperature of a boil-off gas discharged from each compressor30is not relatively high or when discharge of a high-temperature boil-off gas is required, a bypass line (reference numeral not shown) may be provided at a boil-off gas liquefaction line L20so that a boil-off gas may bypass an intercooler60.

A bypass line is provided in a boil-off gas liquefaction line L20so that a compressed boil-off gas bypasses an intercooler60. For example, a bypass line may be provided so that a boil-off gas compressed in a second stage30bbypasses a first intercooler60ato flow into a compressor third stage30c.

A valve (reference numeral not shown) may be provided at a bypass line, and the opening degree of the valve may be adjusted depending on the load of a compressor second stage30bor the like or the temperature conditions of a boil-off gas or the like. However, of course, even when a boil-off gas compressed in a compressor30bypasses an intercooler60along the bypass line, a gas-phase boil-off gas generated within the intercooler60may be transferred toward the compressor30.

In the present embodiment, a compressor30is not limited to three stages30c, and it may have a two-stage structure or a multi-stage structure of four or more stages. However, in the present embodiment, a boil-off gas may be allowed to pass through an intercooler60in the process of being compressed.

A condenser40cools a compressed boil-off gas and re-liquefies at least a part thereof. At this time, the condenser40may re-liquefy the boil-off gas, but it is noted that this does not exclude a situation in which the boil-off gas is not re-liquefied at all or only a part of the boil-off gas is re-liquefied due to various factors during actual operation.

This is because substances with different boiling points are mixed in a boil-off gas. For example, in the case of LPG which includes propane and butane as main ingredients but also includes ethane or the like, the boiling point of ethane is lower than that of propane/butane, and thus some ingredients such as ethane may not be re-liquefied.

A condenser40is provided downstream of a compressor30provided in multiple stages, and uses various refrigerants (e.g., sea water, fresh water, glycol water, nitrogen, LNG, LPG, propane, R134a, CO2, etc.) without limitation to cool a boil-off gas.

A condenser40may lower the temperature of a boil-off gas compressed in a compressor30, but may not lower the temperature to the boiling point of the boil-off gas at atmospheric pressure. This is because the boiling point increases as the boil-off gas is compressed by the compressor30.

However, a condenser40may adjust the cooling temperature of a boil-off gas in consideration of the pressure of the boil-off gas discharged from a compressor30in the final stage (for example, a third stage30c).

A receiver50temporarily stores a boil-off gas liquefied in a condenser40. A boil-off gas liquefaction line L20is provided between a condenser40and a liquefied gas storage tank10to transfer a cooled boil-off gas to a liquefied gas storage tank10, and a receiver50may be disposed on the boil-off gas liquefaction line L20downstream of the condenser40and upstream of an intercooler60.

Similar to a buffer20, a receiver50may have a gas-liquid separation function and may transfer a liquefied boil-off gas among cooled boil-off gases to an intercooler60. However, a receiver50may store a non-liquefied boil-off gas among cooled boil-off gases without discharging to the outside, and in this case, as the internal pressure of the receiver50increases, when the pressure is reduced by a depressurizing valve61, which will be described later, the cooling effect of the boil-off gas may be improved.

Of course, in the present embodiment, various modifications are possible such that the receiver50may transfer a non-liquefied boil-off gas (non-condensable gas) to a vent header or a liquefied gas storage tank10through a vent line L23, or the non-liquefied boil-off gas may be transferred between a compressor third stage30cand a condenser40or the like.

However, a receiver50may be omitted, and in this case, a boil-off gas cooled in a condenser40may be transferred to an intercooler60without separate gas-liquid separation.

An intercooler60heat exchanges between a part of a boil-off gas liquefied in a condenser40and the rest. An intercooler60is branched from a boil-off gas liquefaction line L20upstream of the intercooler60and is connected to a first boil-off gas branch line L21aprovided with a depressurizing valve61, and it is also provided with a cooling flow path62which allows the boil-off gas liquefied in the condenser40to pass therethrough.

An intercooler60has a space for accommodating a boil-off gas depressurized by a depressurizing valve61, and a first boil-off gas branch line L21ais provided to have an open shape within the intercooler60to fill the intercooler60with a boil-off gas, and a cooling flow path62is provided to pass through the inside of the intercooler60.

A depressurizing valve61provided at a first boil-off gas branch line L21areduces the pressure of a boil-off gas branched upstream of an intercooler60after being cooled by a condenser40. A depressurizing valve61cools a boil-off gas by depressurizing it with a Joule-Thomson valve or an expander (Joule-Thomson effect), and thus the depressurizing valve61may liquefy a boil-off gas at a higher rater compared to the boil-off gas cooled by a condenser40(or supercooling).

Therefore, an intercooler60may allow a cooling flow path62of a boil-off gas liquefaction line L20to pass through the inside of a boil-off gas liquefied by depressurization, thereby enabling stable liquefaction through non-contact heat exchange between boil-off gases without a separate refrigerant. In this respect, an intercooler60may be referred to as a heat exchanger and, for example, it may be considered as a bath type heat exchanger. At this time, the cooling flow path62may be provided in a coil shape inside a liquefied boil-off gas to improve liquefaction efficiency.

When two or more intercoolers60are provided, a depressurizing valve61may branch off from the upstream of each intercooler60at a boil-off gas liquefaction line L20and may be provided at each first boil-off gas branch line L21aconnected to the intercooler60. In addition, an intercooler60may implement the role of an intermediate stage cooler of a compressor30upstream of a condenser40. An intercooler60may be connected to an intermediate stage of a compressor30at a boil-off gas liquefaction line L20to cool a boil-off gas compressed by a part of the plurality of compression stages of the compressor30using a decompressed boil-off gas, and it may transfer a boil-off gas generated by heat exchange to the compressor30.

An intercooler60may be provided with a compressed gas inlet (reference numeral not shown) that is connected to a boil-off gas liquefaction line L20upstream of a condenser40to allow a boil-off gas compressed by at least a first stage30aof a compressor to flow into the inside. A compressed gas inlet may be provided at a position higher than the level of a liquid-phase boil-off gas stored inside an intercooler60, which is to prevent unnecessary vaporization of a liquefied boil-off gas.

In addition, an intercooler60may be provided with a depressurized gas inlet (reference numeral not shown) that is connected to a first boil-off branch line L21ato allow a liquefied boil-off gas to flow into the inside, and it may be provided at a position higher than the level of the boil-off gas within the intercooler60.

Therefore, a boil-off gas introduced through a compressed gas inlet may be cooled/liquefied while contacting with a boil-off gas liquefied by depressurization. Through this contact-type heat exchange, cooling in a compressor intermediate stage30may be implemented by an intercooler60.

Inside an intercooler60, a partition wall (reference numeral not shown) facing a compressed gas inlet may be provided, and the partition may prevent a compressed boil-off gas from immediately escaping to a next compressor30without being cooled within an intercooler60.

In the present embodiment, a total of two intercoolers60may be provided. A first intercooler60amay be provided upstream of two intercoolers60based on a boil-off gas flow downstream of a condenser40and may be provided so that a boil-off gas is introduced between a compressor second stage30band a compressor third stage30c.

In addition, a second intercooler60bmay be provided downstream of the two intercoolers60based on the boil-off gas flow downstream of the condenser40, and may be provided so that a boil-off gas is introduced between a compressor first stage30aand a compressor second stage30b.

Therefore, a boil-off gas may be introduced to a compressor first stage30a-a second intercooler60b-a compressor second stage30b-a first intercooler60a-a compressor third stage30c-a condenser40along a boil-off gas liquefaction line L20(or bypass an intercooler60), and the boil-off gas condensed in the condenser40may be returned to a liquefied gas storage tank10through the first intercooler60a-the second intercooler60b-a pressure control valve70along the boil-off gas liquefaction line L20.

In this case, the boil-off gas cooled in the condenser40at 20 to 30 bar and around 40 degrees may undergo almost no change in pressure while passing through the first intercooler60a, and the temperature may drop below 30 degrees, and as it further passes through the second intercooler60b, the temperature may fall below 30 degrees with almost no change in pressure.

Afterwards, when the pressure drops to a level similar to the internal pressure of the liquefied gas storage tank10by the pressure control valve70, the boil-off gas may be cooled to approximately a temperature lower than the boiling point at atmospheric pressure, so it may be finally re-liquefied to be returned to the liquefied gas storage tank10.

In the present embodiment, in replacement of the first boil-off gas branch line L21aor together with the first boil-off gas branch line L21a, a second evaporation gas branch line L21bmay be used. The second boil-off gas branch line L21bhas a difference in branch point in the boil-off gas liquefaction line L20compared to the first boil-off gas branch line L21a.

In other words, the second boil-off gas branch line L21bmay be provided to branch at one point downstream of the second intercooler60bso that branch may each be connected toward the first intercooler60aand the second intercooler60b.

However, a second boil-off gas branch line L21bmay be provided with a depressurizing valve61in the same way as a first evaporation gas branch line L21a, so that a boil-off gas cooled while passing through two intercoolers60may be further cooled by depressurization and then transferred to each intercooler60.

In the present embodiment, both boil-off gas branch lines L21may be included and at least one boil-off branch line L21may be included. When both boil-off gas branch lines L21are included, flow in each boil-off gas branch line L21may be controlled according to various variables such as the temperature or flow rate of the boil-off gas.

A pressure control valve70is provided downstream of a second intercooler60band upstream of a liquefied gas storage tank10in a boil-off gas liquefaction line L20, and it controls the pressure of a boil-off gas according to the internal pressure of the liquefied gas storage tank10, for example, it depressurizes the boil-off gas.

A pressure control valve70may depressurize a boil-off gas of 20 to 30 bar to around 1 bar to correspond to the internal pressure of a liquefied gas storage tank10, and it may be a Joule-Thompson valve, etc., in the same way as or in a similar way to a pressure reducing valve61.

When a pressure control valve70depressurizes a boil-off gas, the temperature of the boil-off gas decreases due to pressurization. For example, a boil-off gas that has passed through an intercooler60twice along a boil-off gas liquefaction line L20has a temperature below zero (for example, around −4 degrees), and as it passes through a pressure control valve70, the temperature of the boil-off gas may decrease to around −40 degrees.

A pressure control valve70may be provided alone or serially in a plural number, and this may vary depending on final compression pressure of a multi-stage compressor30.

A liquefied gas pump90pressurizes a liquefied gas in a liquefied gas storage tank10. A liquefied gas storage tank10may be provided with a liquefied gas supply line L31for supplying a liquefied gas to a demand site (engine, etc.), and the liquefied gas pump90transfers a liquefied gas through the liquefied gas supply line L31.

In addition to supplying a liquefied gas to a demand site, a liquefied gas pump90may also supply a liquefied gas to an intercooler60. This is to prevent generation of a non-condensable gas. First, the generation of a non-condensable gas and problems resulting therefrom will be described below.

As mentioned earlier, a boil-off gas may be LPG. In this case, the boil-off gas may be a mixture of a first substance and a second substance with different boiling points. For example, the boil-off gas may be a mixture of ethane, propane, butane, etc. in ascending order of the boiling point.

A boil-off gas is compressed in a compressor30, condensed in a condenser40, and then divided and introduced into an intercooler60through a receiver50, and a gas-phase boil-off gas generated within the intercooler60is again circulated to the compressor30. That is, substances that are not liquefied in the intercooler60(in particular, ethane, etc. as a first substance with a relatively low boiling point) are continuously circulated.

As system operation time elapses, when a first substance repeatedly circulates through a compressor30-a condenser40-a receiver50-an intercooler60, the proportion of the first substance may increase compared to the boil-off gas circulating the condenser40, thereby significantly decreasing liquefaction efficiency in the condenser40.

To prepare for this, it is needed to block the discharge of the receiver50at a certain point according to the proportion of the first substance in the boil-off gas, forcibly raise the discharge pressure of the compressor30, sufficiently liquefy the first substance in the condenser40, and then allow a flow of the boil-off gas so that the proportion of the first substance in the gas-phase boil-off gas transferred from the intercooler60to the compressor30is lowered again. This operation may be referred to as a non-condensable gas processing mode.

Since a non-condensable gas processing mode may be a factor that rapidly reduces re-liquefaction efficiency, in the present embodiment, a liquefied gas may be transferred into an intercooler60to prevent vaporization of a first substance within an intercooler60so that the operation of the non-condensable gas processing mode may be omitted.

Specifically, a liquefied gas pump90may supply a liquefied gas through a liquefied gas transfer line L30that branches off from a liquefied gas supply line L31and connects to an intercooler60, and it may supply the liquefied gas to the intercooler60to liquefy a gas-phase boil-off gas in the intercooler60.

A part of a liquid-phase boil-off gas condensed in a condenser40may be depressurized by a depressurizing valve61and stored inside an intercooler60, and the intercooler60may passes the rest of the condensed liquid-phase boil-off gas through the inside to mutually heat exchange with the boil-off gas. At this time, the liquefied gas pump90may inject the liquefied gas into the intercooler60, thereby lowering the temperature of a part of the boil-off gas stored inside the intercooler60.

In addition, as the liquefied gas is injected to the intercooler60, the rest of the boil-off gas passing through the inside of the intercooler60is cooled by a part of the boil-off gas stored in the intercooler60and further cooled due to the mixing of the liquefied gas. Therefore, the cooling effect may be increased upon heat exchange between boil-off gases by the intercooler60.

In other words, an intercooler60may utilize a liquefied gas transferred by a liquefied gas pump90in cooling (prevent evaporation) a part of the boil-off gas injected to the inside of the intercooler60, and also utilize it as a refrigerant for a boil-off gas flowing in a cooling flow path62.

In particular, the present embodiment has an effect of suppressing continuous circulation of a first substance in the sense that a liquefied gas pump90transfers a liquefied gas to an intercooler60, thereby limiting the evaporation amount of the first material within the intercooler60to within a preset value.

Specifically, a liquefied gas pump90may transfer a liquefied gas to an intercooler60to reduce the flow rate of a first substance transferred from the intercooler60to a compressor30so that the proportion of the first substance in the boil-off gas flowing through the condenser40is within a preset value.

Since a liquefied gas pump90may operate continuously to supply a liquefied gas to demand site through a liquefied gas supply line L31, transfer of the liquefied gas to an intercooler60may be controlled by opening and closing a valve (reference numeral not shown) provided at a liquefied gas delivery line L30.

Alternatively, when the proportion of the first substance in the boil-off gas flowing through the condenser40is more than or equal to a preset value, the liquefied gas pump90may be controlled to transfer the liquefied gas to the intercooler60. This control may be used in cases where a liquefied gas fuel is not supplied (when anchored, etc.).

A fuel supply portion100processes a liquefied gas supplied from a liquefied gas pump90to a demand site in accordance with the requirements of the demand site. A fuel supply portion100may include a high-pressure pump (not shown), a heat exchanger (not shown) or the like, and in addition, it may be provided with various components to meet the requirements of the demand site, such as the temperature, pressure, and flow rate of the liquefied gas.

A fuel supply portion100may transfer a liquefied gas to a demand site through a liquefied gas supply line L31, or it is also possible to transfer a re-liquefied boil-off gas to a demand site. To this end, a boil-off gas liquefaction line L20may branch at an appropriate point and be connected to the liquefied gas supply line L31, and a boil-off gas may be supplied to a demand site together with a liquefied gas or alone.

In addition, a demand site may discharge an unconsumed surplus liquefied gas among the supplied liquefied gas, and the surplus liquefied gas discharged from the demand site may be recovered to a fuel supply portion100(particularly upstream of a high-pressure pump). To this end, a liquefied gas recovery line (not shown) may be provided as a liquefied gas supply line L31at a demand site.

In this way, in the present embodiment, to solve the problem that the liquefaction efficiency is decreased as a first substance with a low boiling point, such as ethane, continuously circulates between an intercooler60, a compressor30, and a condenser40upon re-liquefying a boil-off gas, a liquefied gas is injected to an intercooler60to effectively suppress evaporation of the first material, thereby ensuring sufficient re-liquefaction efficiency.FIG.2shows a conceptual diagram of a boil-off gas re-liquefying system according to a second embodiment of the present invention.

Hereinafter, the description will focus on the differences between the present embodiment and the previous embodiment, and parts omitted from the description will be replaced with the previous content.

Referring toFIG.2, unlike the previous embodiment, a boil-off gas re-liquefying system1according to a second embodiment of the present invention has a configuration that separates a non-condensable gas and processes it separately.

In other words, in the present embodiment, to improve the problem that the liquefaction efficiency is decreased as a first substance continuously circulates between an intercooler60, a compressor30, and a condenser40, a non-condensable gas separated from a receiver50is separately processed, thereby reducing the proportion of the first material transferred from the intercooler60to the compressor30and preventing the re-liquefaction efficiency from decreasing due to the non-condensable gas.

Specifically, in the present embodiment, a non-condensable gas separated and discharged from a receiver50may be cooled in an additional intercooler60c(which may also be referred to as a heat exchanger). The additional intercooler60cwill be described in detail below, and a non-condensable gas processing line L22through which a non-condensable gas flows may be provided from the receiver50to the additional intercooler60c.

An additional intercooler60cuses at least a part of a liquid-phase boil-off gas transferred from a receiver50to cool a non-condensable gas separated from the receiver50. In the case of the above-described intercooler60, a part of the boil-off gas condensed in a condenser40is depressurized to cool the rest of the boil-off gas, but the additional intercooler60cmay cool the non-condensable gas separated from the receiver50through at least a part of the condensed boil-off gas.

At this time, the additional intercooler60cmay be provided to replace the first intercooler60a, or the additional intercooler60cmay also be provided together with the first and second intercoolers60. However, the explanation below assumes the former case.

An additional intercooler60cmay depressurize a liquid-phase boil-off gas transferred from a receiver50with a depressurizing valve61and the store it in the inside, and it is provided to allow a non-condensable gas to pass through a cooling flow path62in the inside to heat exchange with the liquid-phase boil-off gas. At this time, the non-condensable gas passing through the inside of the additional intercooler60cmay be cooled by the liquid-phase boil-off gas and then transferred to a liquefied gas storage tank10.

In addition, similar to the above-described first intercooler60a, an additional intercooler60cmay transfer a gas-phase boil-off gas generated internally during heat exchange to a compressor30. Therefore, an additional intercooler60cmay also be used to implement intermediate cooling of a compressor30.

In addition or alternatively, an additional intercooler60cmay transfer a gas-phase boil-off gas generated by heat exchange to a liquid-phase boil-off gas flowing from an intercooler60to a liquefied gas storage tank10. That is, the additional intercooler60cmay allow the gas-phase boil-off gas to be injected to a boil-off liquefaction line L20, and in this case, the gas-phase boil-off gas transferred from the additional intercooler60cto the boil-off liquefaction line L20be joined around a point at which a liquid phase is introduced to the boil-off liquefaction line L20from a gas-liquid separator, which will be described later.

Even when a non-condensable gas separated from a receiver50is cooled by a boil-off gas while passing through the inside of an additional intercooler60c, it may not be completely re-liquefied, so a gas-liquid separator80may be provided to prepare for this, and a non-condensable gas processing line L22may extend from a receiver50, pass through an additional intercooler60c, and then be connected to a gas-liquid separator80. The gas-liquid separator80will be described later.

A gas-liquid separator80receives a cooled non-condensable gas and performs gas-liquid separation. A gas-liquid separator80is provided on a non-condensable gas processing line L22, and it may be provided between an additional intercooler60cand a liquefied gas storage tank10based on the flow of the non-condensable gas.

As mentioned earlier, a non-condensable gas separated from a receiver50may be at least partially liquefied by a boil-off gas in an additional intercooler60c, but a gas phase may be partially present, and when the gas phase is injected to a liquefied gas storage tank10, the effect of reducing the proportion of a first substance in a condenser40may be reduced.

Therefore, a gas-liquid separator80may transfer only a liquid phase of a cooled non-condensable gas to a liquefied gas storage tank10, and a gas phase may be discharged to the outside (vent header, etc.) through a vent line L23or supplied to a separate demand site.

In this way, the present embodiment may solve the problem that liquefaction efficiency of a condenser40is reduced as continuous circulation of a first substance occurs in the process of re-liquefying a liquefied gas by performing cooling treatment of a non-condensable gas that may be separated from a receiver50with a boil-off gas. Therefore, the present embodiment may omit or reduce the need to separately operate a non-condensable gas processing mode, and stable liquefaction performance may be maintained.

In addition to the embodiments described above, the present invention encompasses combinations of the above embodiments and embodiments resulting from a combination of at least one of the above embodiments and known techniques.

Although the present invention has been described above in detail through specific embodiments, these are for specifically explaining the present invention, and the present invention is not limited thereto, and it is clear that modifications and improvements thereof are possible by those skilled in the art within the technical spirit of the present invention.

All simple modifications or changes of the present invention fall within the scope of the present invention, and the specific scope of protection of the present invention will be made clear by the appended claims.