KR102595971B1 - System and Method for Re-liquefying Boil-Off Gas - Google Patents

Abstract

A method of re-liquefying boil-off gas is disclosed, in which boil-off gas is re-liquefied through a compression process using a first compressor, a cooling process using a heat exchanger, and a decompression process using a pressure reducing device.
In the method of evaporating gas reliquefaction, the heat exchanger uses a fluid circulating in a circulation cycle as a refrigerant, and the fluid circulating in the circulation cycle includes a compression process by a second compressor, a primary cooling process by the heat exchanger, and a first cooling process by the heat exchanger. 1 After going through the secondary cooling process by the expander and the tertiary cooling process by the second expander, the fluid is used as a refrigerant in the heat exchanger and flows into the first expander after going through the primary cooling process by the heat exchanger. The flow rate is controlled by a control valve.

Description

{System and Method for Re-liquefying Boil-Off Gas}

The present invention is an improved system and method for re-liquefying the boil-off gas itself using it as a refrigerant.

Recently, the consumption of liquefied gas such as liquefied natural gas (LNG) is rapidly increasing worldwide. Liquefied gas, which is made by liquefying gas at low temperature, has a much smaller volume than gas, so it has the advantage of increasing storage and transportation efficiency. In addition, liquefied gas, including liquefied natural gas, can remove or reduce air pollutants during the liquefaction process, so it can be viewed as an eco-friendly fuel with low emissions of air pollutants during combustion.

Liquefied natural gas is a colorless and transparent liquid obtained by liquefying natural gas containing methane as a main component by cooling it to about -163°C, and has a volume of about 1/600 of that of natural gas. Therefore, when natural gas is liquefied and transported, it can be transported very efficiently.

However, since the liquefaction temperature of natural gas is extremely low at -163°C at normal pressure, liquefied natural gas is sensitive to temperature changes and easily evaporates. For this reason, the storage tank that stores liquefied natural gas is insulated, but external heat is continuously transferred to the storage tank, so during the transportation of liquefied natural gas, liquefied natural gas is continuously naturally vaporized within the storage tank, producing boil-off gas (boil). -Off Gas, BOG) occurs.

Evaporation gas is a type of loss and is an important issue in transportation efficiency. In addition, if evaporation gas accumulates in the storage tank, the pressure inside the tank may increase excessively, and in severe cases, there is a risk of tank damage. Therefore, various methods are being studied to treat boil-off gas generated within the storage tank. Recently, for the treatment of boil-off gas, a method of re-liquefying the boil-off gas and returning it to the storage tank, using the boil-off gas as fuel for ship engines, etc. Methods such as using it as an energy source for consumers are being used.

Methods for re-liquefying the boil-off gas include a method of re-liquefying the boil-off gas by heat-exchanging it with a refrigerant using a refrigeration cycle using a separate refrigerant, and a method of re-liquefying the boil-off gas itself as a refrigerant without a separate refrigerant. There is. In particular, the system employing the latter method is called the Partial Re-liquefaction System (PRS).

Meanwhile, among engines generally used in ships, engines that can use natural gas as fuel include gas fuel engines such as DFDE, X-DF engines, and ME-GI engines.

DFDE consists of a 4-stroke cycle and adopts the Otto Cycle, which injects natural gas with a relatively low pressure of about 6.5 bar into the combustion air inlet and compresses it as the piston rises.

The X-DF engine is composed of two strokes, uses natural gas of about 16 bar as fuel, and adopts the Otto cycle.

The ME-GI engine consists of two strokes and adopts the diesel cycle, which injects high-pressure natural gas around 300 bar directly into the combustion chamber near the top dead center of the piston.

The present invention seeks to provide a system and method for re-liquefying evaporation gas, which can prevent liquid droplets from entering the second expander, causing damage to the second expander or shortening its lifespan.

According to one aspect of the present invention for achieving the above object, there is a method of re-liquefying boil-off gas, in which boil-off gas is re-liquefied by a compression process by a first compressor, a cooling process by a heat exchanger, and a decompression process by a decompression device. , the heat exchanger uses the fluid circulating in the circulation cycle as a refrigerant, and the fluid circulating in the circulation cycle undergoes a compression process by the second compressor, a primary cooling process by the heat exchanger, and secondary cooling by the first expander. process, and after going through the third cooling process by the second expander, it is used as a refrigerant in the heat exchanger, and after going through the first cooling process by the heat exchanger, the flow rate of the fluid flowing into the first expander is controlled by a control valve. A method for re-liquefying boil-off gas is provided.

When the circulation cycle is not overcooled, the control valve can be controlled in real time, and when the circulation cycle is overcooled, the control valve can be controlled in advance.

When the circulation cycle is not subcooled, there is a condition in which the flow rate of the boil-off gas compressed by the first compressor falls below the first set value (hereinafter referred to as 'flow rate condition'), and the flow rate of the boil-off gas compressed by the first compressor is In the case where the condition that the temperature of the fluid cooled by the heat exchanger after being compressed falls below the second set value (hereinafter referred to as 'temperature condition') is not satisfied, and the circulation cycle is supercooled, This may be the case when one or more of the ‘flow rate condition’ and the ‘temperature condition’ are satisfied.

The control valve can be controlled in real time by using the temperature and pressure of the fluid expanded by the second expander as factors.

The boil-off gas is divided into two flows, one flow is sent to the first compressor and the remaining flow is sent to the second compressor, and the flow sent to the second compressor may circulate in the circulation cycle.

The circulation cycle may be a closed loop.

According to another aspect of the present invention for achieving the above object, a first compressor for compressing boil-off gas; a heat exchanger that cools the boil-off gas compressed by the first compressor by exchanging heat; A pressure reducing device that depressurizes the fluid cooled by the heat exchanger; and a circulation cycle in which the fluid used as a refrigerant in the heat exchanger is circulated, wherein the circulation cycle includes a second compressor that compresses the fluid circulating in the circulation cycle; a first expander that secondarily cools the fluid compressed by the second compressor and then first cooled in the heat exchanger; a second expander that thirdly cools the fluid secondaryly cooled by the first expander; And a control valve installed between the heat exchanger and the first expander to adjust the flow rate of the fluid flowing into the first expander, wherein the heat exchanger includes three passages and compresses the fluid by the first compressor. A boil-off gas reliquefaction system is provided in which boil-off gas, fluid cooled by the first expander and the second expander, and fluid compressed by the second compressor exchange heat in the heat exchanger.

The boil-off gas reliquefaction system includes a flow sensor that measures the flow rate of boil-off gas compressed by the first compressor; a temperature sensor that measures the temperature of the fluid cooled by the heat exchanger after being compressed by the first compressor; and a control device that adjusts the opening degree of the control valve according to the flow rate value measured by the flow sensor and the temperature value measured by the temperature sensor.

In the boil-off gas reliquefaction system, the circulation cycle may further include a third compressor and a fourth compressor, and the fluid used as a refrigerant in the heat exchanger after being expanded by the first expander and the second expander is After branching into two flows, one flow is compressed by the third compressor and the remaining flow is compressed by the fourth compressor, the fluid compressed by the third compressor and the fluid compressed by the fourth compressor are may be combined and sent to the second compressor.

The third compressor and the first expander, and the fourth compressor and the second expander may each form a compander.

The boil-off gas re-liquefaction system may include a gas-liquid separator installed downstream of the pressure reducing device to separate the re-liquefied liquefied gas from the boil-off gas remaining in a gaseous state.

The boil-off gas separated by the gas-liquid separator may be sent to the first compressor.

According to the present invention, when there is a risk of overcooling the circulation cycle, the control valve can be controlled in advance to prevent liquid droplets from flowing into the second expander.

1 is a schematic diagram of a boil-off gas reliquefaction system according to a preferred embodiment of the present invention.

Hereinafter, the structure and operation of a preferred embodiment of the present invention will be described in detail with reference to the attached drawings. The present invention can be applied to various applications, such as ships equipped with engines using natural gas as fuel and ships containing liquefied gas storage tanks. Additionally, the following examples may be modified into various other forms, and the scope of the present invention is not limited to the following examples.

1 is a schematic diagram of a boil-off gas reliquefaction system according to a preferred embodiment of the present invention.

Referring to FIG. 1, the boil-off gas reliquefaction system of this embodiment includes a first compressor 210, a heat exchanger 100, a pressure reducing device 300, and a circulation cycle (C). Additionally, the circulation cycle (C) includes a second compressor (220), a first expander (410), a second expander (420), and a control valve (V).

The heat exchanger 100 uses the fluid circulating in the circulation cycle C as a refrigerant to cool the boil-off gas compressed by the first compressor 210 by heat exchange. The first compressor 210 can compress the boil-off gas discharged from the storage tank (T), and the boil-off gas compressed by the first compressor 210 is subjected to a cooling process by the heat exchanger 100 and a pressure reduction device. Part or all of it is reliquefied through a decompression process (300).

The present invention may further include a gas-liquid separator 500 installed downstream of the pressure reducing device 300 to separate the re-liquefied liquefied gas and the boil-off gas remaining in a gaseous state. The liquefied gas separated by the gas-liquid separator 500 can be sent to the storage tank (T), and the boil-off gas separated by the gas-liquid separator 500 can be sent back to the first compressor 210.

The fluid circulating in the circulation cycle (C) may be boil-off gas discharged from the storage tank (T). In addition, the boil-off gas is divided into two flows, one flow is sent to the first compressor 210 and the remaining flow is sent to the second compressor 220, and the boil-off gas sent to the second compressor 220 is cycled ( C) It circulates and can be used as a refrigerant.

Although Figure 1 shows that the circulation cycle (C) is configured as a closed loop, the present invention is not limited to this, and the circulation cycle (C) may be configured as an open loop if necessary. Hereinafter, a case where the circulation cycle (C) is composed of a closed loop will be described.

The heat exchanger 100 includes three flow paths, and the boil-off gas circulating in the circulation cycle (C) undergoes a compression process by the second compressor 220 and a primary cooling process by the heat exchanger 100 (FIG. 1 the lowest flow path), a secondary cooling process by the first expander 410, and a tertiary cooling process by the second expander 420 (fluid flows to the first expander 410 and the second expander 420). It expands and the temperature also decreases) and then is used as a refrigerant in the heat exchanger 100 (middle flow path in FIG. 1). The boil-off gas used as a refrigerant in the heat exchanger 100 is sent back to the second compressor 220 and circulates in a closed loop.

That is, the fluid compressed by the first compressor 210 (the uppermost passage in FIG. 1), the fluid cooled by the first expander 410 and the second expander 420 (the middle passage in FIG. 1), The fluid compressed by the second compressor 220 (lowest flow path in FIG. 1) is heat exchanged in the heat exchanger 100.

However, after being compressed by the second compressor 220, the fluid that has gone through the primary cooling process of the heat exchanger 100 and the secondary cooling process of the first expander 410 forms droplets (for example, re-liquefied liquefied gas). ) may include some. In particular, when the flow rate of the re-liquefied boil-off gas (the boil-off gas compressed by the first compressor 210) is small, the refrigerant circulating in the circulation cycle (C) is converted into the boil-off gas re-liquefied through the heat exchanger 100. Since the cold heat transferred is reduced, there is a risk that the entire circulation cycle (C) will be overcooled.

As the flow rate of the re-liquefied boil-off gas (boil-off gas compressed by the first compressor 210) decreases, the temperature of the fluid flowing into the first expander 410 also decreases, and the temperature of the fluid flowing into the first expander 410 decreases. The temperature of the expanded fluid also drops further. When the fluid expanded by the first expander 410 falls below a certain temperature, the probability that liquid droplets exist in the fluid increases. If droplets mixed with the fluid expanded by the first expander 410 flow into the second expander 420, the impeller of the second expander 420 may be damaged or its lifespan may be shortened.

In the present invention, a control valve (V) is installed between the heat exchanger 100 and the first expander 410, and the flow rate of fluid flowing into the first expander 410 is adjusted by the control valve (V).

A condition in which the flow rate of the boil-off gas compressed by the first compressor 210 falls below the first set value (hereinafter referred to as 'flow rate condition'), and a heat exchanger after being compressed by the first compressor 210 ( 100), when both conditions (hereinafter referred to as 'temperature conditions') that the temperature of the cooled fluid falls below the second set value are not satisfied (Normal Case, that is, the circulation cycle (C) is not supercooled. In this case, the control valve (V) is controlled in real time, and if one or more of the 'flow condition' and 'temperature condition' are satisfied (i.e., when the circulation cycle (C) is supercooled), the control valve (V) is controlled in real time. V) is controlled in advance. When controlling the control valve V in real time, the temperature and pressure of the fluid expanded by the second expander 420 can be used as factors.

In addition, when one or more of the 'flow condition' and 'temperature condition' are satisfied, 'controlling the control valve (V) in advance' means that the fluid circulating in the circulation cycle (C) is the first expander (410). This means that the opening degree of the control valve (V) is adjusted in advance before it flows into the ).

Looking at the flow of fluid circulating in the circulation cycle (C), the fluid passing through the control valve (V) is expanded by the first expander 410 and then sent to the second expander 420, so 'flow rate conditions' and When both 'temperature conditions' are not satisfied (Normal Case), controlling the opening degree of the control valve (V) in real time (for example, the temperature and pressure of the fluid expanded by the second expander 420 (a method of controlling the opening degree of the control valve (V) using Therefore, before adjusting the opening degree of the control valve (V), liquid droplets already mixed in the fluid flow into the second expander (420).

Therefore, in the present invention, when one or more of the 'flow condition' and 'temperature condition' are satisfied and the circulation cycle (C) is expected to be supercooled, the control valve (V) is controlled in advance.

The present invention includes a flow sensor 710 that measures the flow rate of boil-off gas compressed by the first compressor 210, and a flow rate sensor 710 that measures the flow rate of the boil-off gas compressed by the first compressor 210 and then cooled by the heat exchanger 100. A temperature sensor 720 that measures temperature, and a control device 730 that adjusts the opening degree of the control valve (V) according to the flow rate value measured by the flow sensor 710 and the temperature value measured by the temperature sensor 720. More may be included.

When the present invention includes a flow sensor 710, a temperature sensor 720, and a control device 730, the 'flow condition' refers to the condition where the flow rate value measured by the flow sensor 710 falls below the first set value. This means that the 'temperature condition' means a condition in which the temperature value measured by the temperature sensor 720 falls below the second set value, and the control device 730 determines that both the 'flow condition' and the 'temperature condition' are If it is not satisfied (Normal Case), the opening degree of the control valve (V) is controlled in real time, and if one or more of the 'flow condition' and 'temperature condition' are satisfied, the control valve (V) is controlled in advance.

Additionally, the circulation cycle C may further include a third compressor 230 and a fourth compressor 240. When the circulation cycle (C) further includes the third compressor 230 and the fourth compressor 240, the refrigerant is expanded by the first expander 410 and the second expander 420 in the heat exchanger 100. The fluid used is divided into two flows, one flow is compressed by the third compressor 230 and the remaining flow is compressed by the fourth compressor 240, and then the fluid compressed by the third compressor 230 The fluid compressed by the fourth compressor 240 may be combined and sent to the second compressor 220. Additionally, the third compressor 230 and the first expander 410, and the fourth compressor 240 and the second expander 420 may each form a compander.

The present invention is not limited to the above-mentioned embodiments, and it is obvious to those skilled in the art that the present invention can be implemented with various modifications or variations without departing from the technical gist of the present invention. It was done.

T: Storage tank V: Control valve
C: Circulation cycle 100: Heat exchanger
210: first compressor 220: second compressor
230: third compressor 240: fourth compressor
300: pressure reducing device 410: first expander
420: second expander 500: gas-liquid separator
710: Flow sensor 720: Temperature sensor
730: Control device

Claims (12)

In the boil-off gas re-liquefaction method of re-liquefying boil-off gas by a compression process by a first compressor, a cooling process by a heat exchanger, and a decompression process by a pressure reducing device,
The heat exchanger uses the fluid circulating in the circulation cycle as a refrigerant,
The fluid circulating in the circulation cycle undergoes a compression process by the second compressor, a primary cooling process by the heat exchanger, a secondary cooling process by the first expander, and a tertiary cooling process by the second expander. Used as a refrigerant in heat exchangers,
After going through the primary cooling process by the heat exchanger, the flow rate of the fluid flowing into the first expander is controlled by a control valve,
A condition ('flow rate condition') in which the flow rate of the boil-off gas compressed by the first compressor falls below a first set value, and the temperature of the fluid cooled by the heat exchanger after being compressed by the first compressor is set to a second If neither of the conditions for falling below the set value ('temperature condition') are satisfied, after the fluid flows into the second expander, the temperature and pressure of the fluid expanded by the second expander are used as factors. Controls the opening degree of the control valve,
When one or more of the 'flow rate condition' and the 'temperature condition' are satisfied, the opening degree of the control valve is controlled before fluid flows into the first expander, thereby lowering the probability of droplets occurring in the circulation cycle. Boil-off gas re-liquefaction method. delete delete delete In claim 1,
Boil-off gas reliquefaction method in which the boil-off gas is divided into two flows, one flow is sent to the first compressor and the remaining flow is sent to the second compressor, and the flow sent to the second compressor circulates the circulation cycle. . In claim 1 or claim 5,
The evaporation gas reliquefaction method wherein the circulation cycle is a closed loop. A first compressor that compresses the boil-off gas;
a heat exchanger that cools the boil-off gas compressed by the first compressor by exchanging heat;
A pressure reducing device that depressurizes the fluid cooled by the heat exchanger; and
It includes a circulation cycle in which the fluid used as a refrigerant in the heat exchanger is circulated,
The circulation cycle is,
a second compressor that compresses the fluid circulating in the circulation cycle;
a first expander that secondarily cools the fluid compressed by the second compressor and then first cooled in the heat exchanger;
a second expander that thirdly cools the fluid secondaryly cooled by the first expander; and
A control valve installed between the heat exchanger and the first expander to control the flow rate of fluid flowing into the first expander,
The heat exchanger includes three flow paths, and the boil-off gas compressed by the first compressor, the fluid cooled by the first expander and the second expander, and the fluid compressed by the second compressor are supplied to the heat exchanger. Heat is exchanged in
A condition ('flow rate condition') in which the flow rate of the boil-off gas compressed by the first compressor falls below a first set value, and the temperature of the fluid cooled by the heat exchanger after being compressed by the first compressor is set to a second If neither of the conditions for falling below the set value ('temperature condition') are satisfied, after the fluid flows into the second expander, the temperature and pressure of the fluid expanded by the second expander are used as factors. Controls the opening degree of the control valve,
When one or more of the 'flow rate condition' and the 'temperature condition' are satisfied, the opening degree of the control valve is controlled before fluid flows into the first expander, thereby lowering the probability of droplets occurring in the circulation cycle. Boil-off gas reliquefaction system. In claim 7,
A flow sensor that measures the flow rate of boil-off gas compressed by the first compressor;
a temperature sensor that measures the temperature of the fluid cooled by the heat exchanger after being compressed by the first compressor; and
a control device that adjusts the opening degree of the control valve according to the flow rate value measured by the flow sensor and the temperature value measured by the temperature sensor;
Further comprising a boil-off gas reliquefaction system. In claim 7,
The circulation cycle further includes a third compressor and a fourth compressor,
After being expanded by the first expander and the second expander, the fluid used as a refrigerant in the heat exchanger is divided into two flows, one flow being compressed by the third compressor and the remaining flow being compressed by the fourth compressor. After compression, the fluid compressed by the third compressor and the fluid compressed by the fourth compressor are combined and sent to the second compressor. In claim 9,
The third compressor and the first expander, and the fourth compressor and the second expander each constitute a compander. The method of any one of claims 7 to 10,
A boil-off gas re-liquefaction system installed downstream of the pressure reducing device and including a gas-liquid separator that separates the re-liquefied liquefied gas from the boil-off gas remaining in a gaseous state. In claim 11,
The boil-off gas separated by the gas-liquid separator is sent to the first compressor.

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