KR101623171B1 - BOG Reliquefaction System - Google Patents

Abstract

Disclosed is a device for reliquefying boil-off gas. According to the present invention, cold heat for reliquefying boil-off gas can be additionally supplied by using a compressor additionally installed for redundancy design, without the equipment of a high-capacity compressor. Accordingly, the reliquefaction capacity of the boil-off gas can be improved, even if the compressor with the same capacity is installed. The system for relieufying boil-off gas comprises: a first compressor for compressing a portion of the boil-off gas; a second compressor for compressing the other portion of the boil-off gas; a second expanding means for expanding a portion of flow; a heat exchanger for cooling the other portion of the flow; and a first expanding means for expanding a fluid.

Description

{BOG Reliquefaction System}

The present invention relates to an apparatus and method for re-liquefying a vaporized gas, and more particularly, to an apparatus and a method for re-liquefying a vaporized gas and compressing the vaporized gas to supply an evaporated gas to an engine or the like.

Liquefied natural gas (LNG) refers to natural gas extracted from natural gas and then liquefied. The main component is methane and is liquefied at -163 ° C at atmospheric pressure.

These liquefied natural gas are being actively developed for the marine small- and medium-scale gas field, which is thought to have lost marketability, as the demand for energy and the technology for collecting and transporting liquefied natural gas have developed.

Liquefied natural gas collected from the offshore gas field is liquefied at a cryogenic temperature of -163 degrees Celsius, but when it is liquefied, its volume is about 1/600 of that of the gas state. Therefore, to transport liquefied natural gas, As well as the liquefaction and insulation of liquefied natural gas.

However, despite the insulation system, it is impossible to completely insulate from the outside, so that the liquefied natural gas is continuously evaporated in the tank storing the liquefied natural gas, and the liquefied natural gas evaporating in this way is called the boil off gas (BOG ). If the evaporation gas continues to accumulate in the tank storing the liquefied natural gas, the pressure inside the tank increases, and the stability of the tank lowers, and the generation of the evaporation gas accelerates due to the internal flow of the tank due to the marine environment . Therefore, it is necessary to provide an evaporative gas remelting facility for re-liquefying the evaporated gas and returning it to the tank.

As a method for re-liquefying such evaporation gas, there is a method of re-liquefying the evaporation gas by heat exchange with the refrigerant by providing a separate refrigerant, and a method of re-liquefying the evaporation gas by using the evaporation gas itself as a refrigerant without any refrigerant. Particularly, the system adopting the latter method is called a Partial Re-liquefaction System (PRS).

In this partial remanufacturing system, a compressor is required, which serves to compress the evaporated gas discharged from the tank storing the liquefied natural gas.

Therefore, depending on the capacity of the compressor for compressing the evaporation gas, the evaporation gas re-liquefaction capacity of the partial re-liquefaction system is determined. However, there is a problem in that when the partial liquefaction system is operated, when the amount of the evaporated gas discharged from the tank becomes larger than expected, there is a problem that the evaporated gas can not be immediately treated due to the limit of the capacity of the compressor.

Accordingly, an object of the present invention is to provide an apparatus and method for liquefying evaporated gas that can exhibit improved liquefaction performance of evaporated gas compared to a conventional partial liquefaction system even when the same compressor is installed.

According to an aspect of the present invention, there is provided a storage tank for storing liquefied natural gas; A cold recovery unit provided downstream of the storage tank and supplied with evaporative gas generated in the storage tank; A compressor disposed downstream of the cold / hot water recovering unit and compressing the evaporated gas discharged from the cold / hot water recovering unit; A first expansion means provided downstream of the compressor for expanding a part of the evaporated gas discharged from the cooler; And a gas-liquid separator provided downstream of the first expansion means and separating the gas-liquid mixture, which is decompressed and discharged from the first expansion means, into gas and liquid; Wherein the evaporation gas is a first stream in which evaporation gas generated in the storage tank and gas exhausted from the gas-liquid separator are supplied to the cold / hot heat recovery unit; A second flow which is discharged from the cold / hot water collection unit and supplied to the fuel consumer; A third flow branching from the second flow downstream of the compressor; A fourth flow branching from the second flow downstream of the compressor and supplied to the cold heat recovery unit; Wherein the first flow and the fourth flow are heat-exchanged in the cold / hot water recovering unit, and the compressors are provided in parallel and in parallel with each other.

A heat exchanger provided downstream of the compressor and upstream of the first expansion means for cooling the third flow discharged from the compressor; A second expansion means provided downstream of the heat exchanger for expanding and further cooling the third flow; Wherein the evaporated gas is supplied from the cold heat recovery unit to the heat exchanger; Wherein the third flow discharged from the second expansion means can cool the third flow before being supplied to the fifth flow and the second expansion means through heat exchange in the heat exchanger.

In the compressor, the second flow may be compressed to 10 to 100 bar.

A cooler provided downstream of the compressor for cooling the compressed evaporative gas; Wherein the cooler is capable of cooling the second flow using seawater or air.

According to another aspect of the present invention, there is provided a method for re-liquefying an evaporative gas generated in a storage tank, the method comprising: controlling the amount of evaporative gas discharged from the storage tank, There is provided a method for evaporating gas re-liquefaction which compresses all the evaporated gas discharged from the storage tank by driving a compressor provided to satisfy redundancy.

According to the present invention, it is possible to additionally supply cold heat to re-liquefy the evaporated gas without increasing the capacity of the compressor in the partial re-liquefaction system for re-liquefying the evaporated gas, thereby improving the liquefaction performance.

Also, depending on the situation such as the discharge amount of evaporated gas, the partial refueling system can be driven while the operation of some compressors is interrupted, so that the flow control of the refrigerant flow rate and the cooling / heating can be effected.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram showing an apparatus for evaporating gas re-liquefaction according to an embodiment of the present invention; FIG.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention will be described with reference to the embodiments shown in the drawings, but it is to be understood that various modifications and equivalent embodiments may be made by those skilled in the art without departing from the scope of the present invention. Therefore, the true scope of protection of the present invention should be defined only by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram showing an apparatus for evaporating gas re-liquefaction according to an embodiment of the present invention; FIG. Meanwhile, the evaporation gas referred to in the present invention includes both evaporation gas generated by vaporization of LNG and evaporation gas generated by vaporization of LPG.

As shown in FIG. 1, an apparatus for liquefying evaporated gas according to an embodiment of the present invention includes a storage tank 1 for storing liquefied gas, a cold / hot water recovery unit 10 provided downstream of the storage tank 1, And a first compressor (20) and a second compressor (22) provided downstream of the cold / hot water recovering unit (10). The evaporated gas, which is naturally generated in the storage tank 1 and then discharged, flows through the evaporation gas supply line L 1 and is supplied to the fuel consumer 80. The cold / hot water recovering unit (10) is provided in the evaporation gas supply line (L1) and can recover cold and heat from the evaporated gas immediately after being discharged from the storage tank (1). The cold / hot water recovering unit 10 receives the evaporated gas generated in the storage tank and the gas discharged from the gas-liquid separator 70, which will be described later. The first compressor (20) and the second compressor (22) compress the evaporated gas discharged from the cold / hot water recovering unit (10). The first compressor (20) and the second compressor (22) are provided in parallel. The first compressor (20) and the second compressor (22) may be compressors of the same performance. The second compressor (22) may be a compressor to satisfy the redundancy design of the first compressor (20). The first compressor 20 may be installed on the evaporation gas supply line L1. The second compressor 22 is connected to the redundancy line L2 branched from the evaporation gas supply line L1 upstream of the first compressor 20 and connected to the evaporation gas supply line L1 downstream of the first compressor 20 . Meanwhile, in the embodiment of the present invention, the number of compressors is two, but it is merely an example, and three or more compressors may be provided in parallel. In the compressors 20 and 22, the evaporation gas can be compressed to 10 to 100 bar.

In addition, the apparatus for evaporating gas re-liquefaction according to an embodiment of the present invention may further include a cooler 30, 32 provided downstream of the compressors 20, 22 for cooling the compressed evaporative gas. The cooler includes a first cooler 30 provided downstream of the first compressor 20 on the evaporation gas supply line L1 and a second cooler 30 provided on the downstream side of the second compressor 22 on the redundancy line L2. 32). Although not shown, the coolers 30 and 32 can cool the evaporated gas compressed by the compressors 20 and 22 through heat exchange with seawater, fresh water or air introduced from the outside. Meanwhile, the compressors 20 and 22 may be multi-stage compressors, respectively. Thus, although not shown in FIG. 1, when the first compressor 20 and the second compressor 22 are multi-stage compressors, a plurality of compressors may be provided in series in the compressors 20 and 22. 1 illustrates compressing the evaporated gas to a pressure required by the one of the compressors 20 and 22 to the demand point of the fuel consumer 80. However, when the compressors 20 and 22 are multi-stage compressors as described above, Can be compressed a plurality of times by a plurality of compression units. Meanwhile, when the compressors 20 and 22 are multi-stage compressors, a plurality of coolers 30 and 32 may be provided in series in the compressor.

The apparatus for liquefying vaporized gas according to an embodiment of the present invention includes a first expansion means 50 (hereinafter, referred to as " first expansion means ") that is provided downstream of the coolers 30 and 32 and expands a part of evaporated gases discharged from the coolers 30 and 32 And a gas-liquid separator (70) provided downstream of the first expansion means (50) and separating the gas-liquid mixture, which is decompressed and discharged from the first expansion means (50), into gas and liquid. The first expansion means (50) includes all means capable of producing a gas-liquid mixture from the evaporating gas by expanding and decompressing and cooling the evaporating gas. The first expansion means 50 may be, for example, a Joule-Thomson valve.

The first expansion means 50 and the gas-liquid separator 70 are provided on the evaporation gas return line L3 branched from the downstream side of the first compressor 20 of the evaporation gas supply line L1 and returning to the storage tank 1 Respectively. The evaporation gas flowing through the evaporation gas return line L3 can be cooled through heat exchange with the evaporation gas just after being discharged from the storage tank 1 while passing through the cold heat recovery unit 10. [

The liquid separated in the gas-liquid separator 70 returns to the storage tank 1 through the vapor return line L3 and the gas separated in the gas-liquid separator 70 is discharged from the gas-liquid separator 70 to the cold / And is supplied to the cold heat recovery unit 10 through the gas mixing line L4 extending to the upstream evaporation gas supply line L1.

1, the apparatus for evaporating gas re-liquefaction according to an embodiment of the present invention includes a heat exchanger 40 (not shown) provided downstream of the coolers 30 and 32 and upstream of the first expansion means 50, And a second expansion means (60) provided downstream of the heat exchanger (40).

The heat exchanger 40 is provided on the upstream side of the first expansion means 50 provided in the evaporation gas return line L3 to further cool the evaporated gas that has been primarily cooled while passing through the cold heat recovery unit 10 .

Is branched from the evaporation gas supply line L1 downstream of the first compressor 20 and connected to the evaporation gas supply line L1 upstream of the first compressor 20 in order to further cool the evaporation gas in the heat exchanger 40 The evaporated gas compressed in the first compressor (20) through the recycle line (L5) is supplied to the heat exchanger (40).

The recirculation line (L5) is provided with a second expansion means (60). The evaporated gas compressed in the first compressor (20) is depressurized by the second expansion means (60) and the temperature is lowered. The evaporated gas whose temperature has been lowered through the second expansion means is supplied to the fifth flow 108 and the second expansion means 60, which are supplied through the heat exchange in the heat exchanger 40 via the evaporation gas return line L3, Thereby cooling the third stream 104.

And a fuel demanding unit 80 provided downstream of the compressors 20 and 22 and receiving a part of the evaporated gas discharged from the compressors 20 and 22. The fuel demanding unit 80 may be an engine, a generator, or the like, which is driven by fuel.

Meanwhile, the main flow of the evaporating gas is defined to facilitate the operation of the apparatus for evaporating the gas re-liquefaction according to an embodiment of the present invention. That is, in the present specification, the flow in which the evaporated gas generated in the storage tank 1 and the gas discharged from the gas-liquid separator 70 are supplied to the cold heat recovery unit 10 is referred to as a first flow 100, a cold heat recovery unit 10 The flow from the compressors 20 and 22 to the compressors 20 and 22 and from the compressors 20 and 22 to the fuel consumer 80 is referred to as the second flow 102, The flow branched from the first flow 102 to the heat exchanger 40 is branched from the second flow 102 downstream of the third flow 104 and the compressors 20 and 22 to be supplied to the cold heat recovery unit 10 The fourth flow 106 and the flow from the cold heat recovery unit 10 to the heat exchanger 40 are defined as the fifth flow 108. The first stream 100 becomes the second stream 102 while passing the cold heat recovery unit 10 and the fourth stream 106 becomes the fifth stream 108 as it passes the cold heat recovery unit 10. [

As shown in FIG. 1, an apparatus for regenerating a vaporized gas according to an exemplary embodiment of the present invention may include a plurality of valves for controlling the flow rate of a liquid or gaseous vaporized gas. That is, the third stream 104 is supplied to the heat exchanger 40 on the upstream side of the compressors 20 and 22, the coolers 30 and 32, the upstream side of the heat exchanger 40, 10, the liquid and gas discharged from the gas-liquid separator as a stream on the line where the fourth stream 106 is supplied to the cold heat recovery unit 10 and downstream of the gas-liquid separator are stored in the storage tank 1 and the cold / The valves may be provided in the form of a liquid or gas (not shown), depending on the conditions such as the amount of evaporated gas discharged from the storage tank 1, And serves to control the flow rate of the evaporative gas in the state.

Hereinafter, operation of an apparatus for evaporating gas re-liquefaction according to an embodiment of the present invention will be described with reference to the drawings.

The gaseous evaporative gas generated in the storage tank 1, which stores the liquefied gas in the liquid state, is supplied to the cold heat recovery unit 10. At this time, the gaseous evaporation gas generated in the storage tank 1 meets the gaseous evaporation gas discharged from the gas-liquid separator 70 to form the first flow 100. Ultimately, the fluid supplied to the cold heat recovery unit 10 is the first flow.

On the other hand, the cold / hot water recovering unit 10 recovers cold heat of the first flow to cool the other evaporated gas. That is, the cold / hot water recovering unit 10 recovers the cold heat possessed by the first flow and supplies the cold heat to the cold / hot water recovering unit 10 during the second flow of pressurized air passing through the compressors 20 and 22, That is, to the fourth flow 106. Thus, in the cold heat recovery unit 10, heat exchange occurs between the first flow and the fourth flow so that the first flow 100 is heated and the fourth flow 106 is cooled. The first stream 100 thus heated becomes the second stream 102 and the cooled fourth stream 106 becomes the fifth stream 108. That is, the first flow 100 and the second flow 102, and the fourth flow 106 and the fifth flow 108 are different only in the amount of state such as temperature and pressure, but are the same flow rate.

The second flow 102 discharged from the cold / hot water collection unit 10 is supplied to the compressors 20 and 22. The second flow 102 is compressed in the compressors 20,22. The pressure to be compressed by the compressors 20 and 22 may vary depending on the pressure required by the fuel consumer 80. For example, when the fuel consumer 80 is a ship propulsion engine and the required pressure of the evaporation gas is 10 to 100 bar The compressors 20 and 22 can compress the evaporation gas to such a requirement.

Meanwhile, when the amount of the cold heat required for resuscitation changes according to the amount of the evaporative gas discharged from the storage tank 1 or the amount of the evaporative gas required by the fuel demanding unit 80 changes, a plurality of compressors 20 and 22 ) May be activated or all of them may be activated. For example, if the amount of evaporative gas to be re-liquefied or the amount of evaporative gas to be supplied to the fuel consumer 80 can be processed even if only one of the plurality of compressors 20, 22 is in operation, And the valves provided downstream may be operated to open and close the first and second compressors 20 and 22 so that evaporative gas is supplied to the first compressor 20 or the second compressor 22 to process the evaporative gas. However, if the amount of evaporative gas to be re-liquefied can be processed by operating all of the compressors 20 and 22, both the upstream and downstream valves of the compressors 20 and 22 can be opened to process the evaporated gas. Therefore, when all of the compressors 20 and 22 are driven, it is possible to additionally supply the cold heat necessary for liquefying the evaporated gas, thereby increasing the liquefaction capacity.

In particular, this effect is maximized when a redundancy design is preliminarily provided with the same configuration in case a part of the configuration fails due to a failure or the like. That is, in many situations where the capacity of the compressor is not used properly, it is not necessary to provide an excessively large capacity compressor for the treatment of the evaporative gas and an adequate capacity compressor is provided so that only the main compressor is driven in the normal environment, In the situation where the evaporative gas can not be processed, the auxiliary compressor, which is provided for the redundant design and has the same capacity as the main compressor, can be operated together to process the excess evaporative gas. Therefore, according to the present invention, it is possible to solve the space securing problem and the cost problem caused by the provision of the large capacity compressor. 1, the first compressor 20 may be a primary compressor, and the second compressor 22 may be a secondary compressor for redundancy design.

On the other hand, the second stream 102 compressed by the compressors 20 and 22 is supplied to the fuel consumer 80. On the other hand, some of the second flows 102 discharged from the compressors 20 and 22 are supplied to the cold heat recovery unit 10 by the fourth flow 106. Another portion of the second stream 102 discharged from the compressors 20,22 is also supplied to the heat exchanger 40 as a third stream 104. [

On the other hand, in the re-liquefaction process of the evaporated gas, the heat exchanger 40 is particularly useful when the compressors 20 and 22 compress the evaporated gas to a relatively low pressure. That is, when the evaporation gas is compressed to a high pressure close to 100 bar in the compressor, the fourth flow 106 is cooled in the cold heat recovery unit 10 to the fifth flow 108, 50 and the gas-liquid separator 70, the re-liquefaction efficiency of the evaporation gas is good, but when the evaporation gas is compressed to a low pressure in the compressor, the re-liquefaction efficiency of the evaporation gas may be lowered. Therefore, when compressing the evaporation gas at a low pressure of, for example, 10 bar at the compressors 20 and 22, the fifth flow 108 is supplied to the heat exchanger 40 before being supplied to the first expansion means 50 It is possible to improve the re-liquefaction efficiency of the evaporated gas.

The third stream 104 supplied to the heat exchanger 40 is discharged from the heat exchanger 40 and then reduced in pressure and cooled by the second expansion means 60 and then supplied to the heat exchanger 40 again, (40). At this time, the third stream 104 initially supplied to the heat exchanger 40 is subjected to heat exchange with the third stream 104, which is reduced in pressure and cooled by the second expansion means 60, and then supplied to the heat exchanger 40 And cooled. The third stream 104 which is fed to the heat exchanger 40 and discharged from the heat exchanger 40 twice in this way is again joined upstream of the compressors 20 and 22 and supplied to the compressors 20 and 22.

The fourth flow 106 cooled by heat exchange with the first flow 100 in the cold heat recovery unit 10 becomes the fifth flow 108 and is supplied to the heat exchanger 40. The fifth stream 108 supplied to the heat exchanger 40 is supplied to the second expansion means 60 after being discharged from the heat exchanger 40 and is supplied to the heat exchanger 40 after being reduced in pressure and cooled, And is cooled by heat exchange with the stream 104. [ Thereafter, the fifth stream 108 discharged from the heat exchanger 40 passes through the first expansion means 50, during which a decompression and cooling process takes place. As a result, the fifth stream 108 becomes a vapor-liquid mixture in which gas and liquid are mixed. The fifth stream 108 discharged from the first expansion means 50 is separated into a gas and a liquid while passing through the gas-liquid separator 70, and the liquid is supplied to the storage tank 1 and the gas is returned to the cold / 10) to repeat the above procedures.

L1: Evaporative gas supply line L2: Redundant line
L3: Evaporative gas return line L4: Gas mixture line
L5: recirculation line 1: storage tank
10: cold / heat recovery unit 20: first compressor
22: second compressor 30: first cooler
32: second cooler 40: heat exchanger
50: first expansion means 60: second expansion means
70: gas-liquid separator 80: fuel consumer
100: first flow 102: second flow
104: third flow 106: fourth flow
108: fifth flow

Claims (9)

A system for re-liquefying evaporative gas generated in a storage tank,
A first compressor for compressing a part of evaporation gas (hereinafter referred to as a fluid) discharged from the storage tank;
A second compressor for compressing another portion (hereinafter, referred to as 'b fluid') of the evaporated gas discharged from the storage tank;
A second expansion means for expanding a portion (hereinafter referred to as 'c fluid') of the flow where the a fluid and the b fluid are merged;
A heat exchanger for cooling another portion (hereinafter referred to as "d fluid") of the flow where the a fluid and the b fluid are merged;
And a first expansion means for expanding the d fluid cooled by the heat exchanger,
Wherein the heat exchanger cools the d fluid by heat exchange of the c fluid expanded by the second expansion means with the refrigerant. The method according to claim 1,
Further comprising a cold / hot water recovering unit for cooling the evaporated gas discharged from the storage tank by heat exchange with the d fluid using refrigerant. The method of claim 2,
Wherein the d fluid is firstly cooled in the cold heat recovery unit, cooled secondarily in the heat exchanger, and then expanded by the first expansion means to re-liquefy. The method according to any one of claims 1 to 3,
The c fluid is supplied to the second expansion means after passing through the heat exchanger,
Wherein the heat exchanger performs heat exchange between the c fluid before passing through the second expansion means, the c fluid after passing through the second expansion means, and the d fluid. The method according to any one of claims 1 to 3,
And the c fluid passing through the second expansion means and the heat exchanger is merged with the evaporated gas discharged from the storage tank. The method of claim 4,
And the c fluid passing through the second expansion means and the heat exchanger is merged with the evaporated gas discharged from the storage tank. The method of claim 6,
Further comprising a gas-liquid separator for separating the re-liquefied liquefied natural gas from the d-fluid that has passed through the first expansion means and the vaporized gas remaining in the gaseous state,
The liquefied natural gas separated by the gas-liquid separator is sent to the storage tank,
Wherein the evaporated gas separated by the gas-liquid separator is merged with the evaporated gas discharged from the storage tank. The method of claim 6,
Wherein the flow other than the c fluid and the d fluid is sent to the fuel demanding place among the flows where the a fluid and the b fluid are merged. The method of claim 8,
Wherein the first compressor and the second compressor compress the evaporation gas to 10 to 100 bar.

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