RU2728305C1 - Liquefied natural gas production system equipped with recondensator - Google Patents

Abstract

FIELD: oil, gas and coke-chemical industries.SUBSTANCE: invention relates to liquefaction of natural gas (LNG). LNG production system includes liquefier (14), which cools and liquefies natural gas by means of coolant supplied from refrigerating unit, storage tank (16) for LNG storage, line (L6) for movement of liquefied natural gas from LNG tank (16), LNG carrier (18), by means of which liquefied natural gas passing through line (L6) is transported, recondenser (17), which recondenses the stripping gas generated by the heat transferred to the liquefied natural gas, by means of a coolant supplied from refrigeration unit (15), and return line (A4), which supplies liquefied natural gas which has been liquefied, into LNG tank (16) from recondensator (17).EFFECT: technical result is enabling possibility of recondensing of the gas without using the exhaust gas compressor and regardless of the LNG liquefaction stage.6 cl, 10 dwg

Description

[Engineering Field]

The present invention relates to a liquefied natural gas (LNG) production system equipped with a recondenser (re-liquefaction condenser) that recondenses (re-liquefies) a boil-off gas (stripping gas).

[Tech tier]

FIG. 6 is a general view of the LNG production system. Natural gas (NG, natural gas) is supplied to step 62 CO ₂ removal in the subsequent stage via the compressor 61. During step 62 CO ₂ removal from natural gas removed CO ₂ using a predetermined solvent. After removal, the SG, from which CO _{2 has} been removed, is fed to the drying stage 63. During the drying step 63, a predetermined post-removal treatment of the PG is carried out. The dried NG is fed to the liquefaction stage 64. In the liquefaction stage 64, the dried LHG is liquefied using liquid refrigerant supplied from the refrigeration system 65. The liquefied natural gas (LNG) is supplied to the LNG storage tank 66. Thereafter, in the LNG loading step 68, LNG is supplied from the LNG storage tank 66 at a predetermined point in time (eg, at the point in time that the LNG is transferred to the tank of a transport vessel, or the like). However, the LNG in the LNG storage tank 66 sometimes vaporizes due to the supplied ambient heat and generates exhaust gas. In addition, when LNG is transferred to the tank of a transport vessel from the LNG storage tank 66, a large amount of exhaust gas is sometimes generated in the LNG loading stage 68. In addition, the pipeline cools as the LNG is moved into the tank of the transport vessel, so that exhaust gas is sometimes generated.

Many LNG storage tanks 66 that are installed at LNG production sites have large capacities, and their design pressure is generally set close to atmospheric pressure for technology and cost reasons. Therefore, it is important to vent the exhaust gas from the LNG storage tank 66 even with a slight increase in pressure. In addition, the pressure increase also occurs due to the piston effect (also called "pushing effect") accompanying the delivery of LNG to the LNG storage tank 66 after the liquefaction stage 64, and therefore the delivery of the LNG to the LNG storage tank 66 causes a regular release OG. In addition, there are also instantaneous losses due to decompression accompanying loading into the LNG storage tank 66 from the liquefier in the liquefaction stage 64. It should be noted that the LNG loss due to instantaneous loss is sometimes approximately 50% of the generated exhaust gas.

However, it is undesirable to vent the exhaust gas into the atmosphere from an environmental and economic point of view, so that the exhaust gas is traditionally returned to the drying stage 63 by means of the compressor 67 and delivered to the liquefaction stage 64 along with the dried NG. As a result, the exhaust gas can be re-liquefied. Alternatively, exhaust gas is sometimes used as a heat source for regenerating drying material or the like in the drying step 63.

However, in the case of re-liquefying the exhaust gas as described above, the total amount of GHG fed to the liquefaction stage 64 from the drying stage 63 contains the recycled exhaust gas. In addition, being used as a fuel gas means that the total amount of LNG that is produced cannot be transferred to the tank of a transport vessel.

In addition, in order to re-liquefy the exhaust gas, it is necessary to bring the exhaust gas to a high pressure state, therefore, during the recirculation, a compressor 67 is needed, which returns the exhaust gas to the drying stage 63. Consequently, a large amount of energy is required to compress the exhaust gas.

In addition, when returning the exhaust gas to the drying stage 63 as described above, the exhaust gas can only be re-liquefied during the production of LNG, so that if the exhaust gas is to be released from the LNG storage tank when no LNG is produced, the exhaust gas must be released into the air. ... That is, the timing for re-liquefaction is limited, and the re-liquefaction step is not flexible.

The LNG production system described in Patent Literature 1 does not provide a way to solve the above problem.

[List of links]

[Patent Literature]

[Patent Literature 1] US Patent No. 2011/0094261

[Summary of the invention]

[Technical problem]

An object of the present invention is to provide an LNG production system comprising a re-condenser that can re-condense an exhaust gas (stripping gas) without the use of an exhaust gas compressor and regardless of the LNG liquefaction stage.

[Solution to the problem]

The first LNG production system according to the present invention comprises

a liquefier that cools and liquefies natural gas using a refrigerant supplied from a refrigeration plant,

an LNG tank that stores liquefied natural gas (LNG) liquefied in a liquefier,

transfer line for transferring liquefied natural gas from the LNG tank,

an LNG transporter located at the next stage of the transfer line and designed to transport liquefied natural gas,

a re-condenser that recondenses (re-liquefies) the stripping gas generated by the heat transferred to the liquefied natural gas using the refrigerant supplied from the refrigeration unit, and

a return line that feeds the liquefied natural gas that has been liquefied to the LNG tank from the recondenser.

The second LNG production system according to the present invention comprises

a liquefier that cools and liquefies natural gas using the refrigerant supplied from the first refrigeration unit,

an LNG tank that stores liquefied natural gas, liquefied in a liquefier,

transfer line for transferring liquefied natural gas from the LNG tank,

an LNG transporter located at the next stage of the transfer line and designed to transport liquefied natural gas,

a re-condenser that recondenses the stripping gas (exhaust gas) generated by the heat transferred to the liquefied natural gas with the refrigerant supplied from the second refrigeration unit, and

a return line that feeds the liquefied natural gas that has been liquefied to the LNG tank from the recondenser.

In the above aspect of the present invention, the refrigerant supplied from the first refrigeration unit and the refrigerant supplied from the second refrigeration unit may be the same refrigerants or may be different refrigerants. For example, as the refrigerant from the first refrigeration unit, a mixture such as hydrocarbon is given, and as the refrigerant from the second refrigeration unit, nitrogen or the like is given.

The third LNG production system according to the present invention comprises

a liquefier that cools and liquefies natural gas using a refrigerant supplied from a refrigeration plant,

an LNG tank that stores liquefied natural gas, liquefied in a liquefier,

transfer line for transferring liquefied natural gas from the LNG tank,

an LNG transporter located at the next stage of the transfer line and designed to transport liquefied natural gas,

a recondenser that is switched to alternately perform the first recondensation process, which consists in liquefying the boil-off gas generated by the heat transferred to the liquefied natural gas with the help of the refrigerant supplied from the refrigeration unit, and the second recondensing process, which consists in liquefying the boil-off gas using the refrigerant supplied from the refrigeration unit and the refrigerant supplied from the refrigerant buffer storage to treat a larger amount of stripping gas than the amount of stripping gas processed during the first recondensation process, and

a return line that feeds the liquefied natural gas that has been liquefied to the LNG tank from the recondenser.

In the above-described aspect of the present invention, the refrigerant supplied from the refrigeration unit and the refrigerant supplied from the coolant buffer storage may be the same refrigerants or may be different refrigerants. For example, a mixture such as hydrocarbon is given as the refrigerant from the refrigeration unit, and nitrogen or the like is given as the refrigerant from the coolant buffer storage.

During the aforementioned first recondenser process, refrigerant may be supplied to the recondenser from the refrigeration unit, and during the second recondenser process, refrigerant from the coolant buffer storage may be supplied to the recondenser in addition to the refrigerant supplied to the recondenser from the refrigeration unit. In case of switching to the second recondensation process from the first recondensing process, the operation of the refrigeration unit can be stopped or the refrigeration unit can operate continuously without stopping.

The condenser may comprise a switch control unit that switches between the first recondensation process and the second recondensation process.

The changeover control unit can switch from the first recondensation process to the second recondensing process in the event that the exhaust gas moves to the LNG conveyor.

The changeover control unit can switch from the first recondenser process to the second recondenser process when the pressure value measured by a pressure gauge located in the LNG tank or on the supply line that supplies exhaust gas to the recondenser becomes equal to or exceeds a predetermined value.

According to the corresponding configurations described above, in the event that the process is carried out over the exhaust gas in an amount within a predetermined range (flow rate per unit time), or if the pressure value is within a predetermined range, set in advance, the first recondensation process is carried out (the exhaust gas liquefaction process with the help of refrigerant from the refrigeration unit), and, in the case of carrying out the technological process over the exhaust gas in an amount exceeding the above-described amount, which is in a predetermined range, or if the pressure value is in a predetermined range, a second technological process of recondensing can be carried out (technological process liquefaction also with the refrigerant supplied from the refrigerant buffer cylinder, whereby liquefaction with the refrigerant supplied from the refrigeration system continues), so that the exhaust gas can be recondensed without the use of an exhaust gas compressor and independently from the LNG liquefaction stage.

In each of the above-described first to third LNG production systems, a predetermined treatment of the aforementioned natural gas supplied to the liquefier may be performed. For example, each of the LNG production systems may include a removal device that removes predetermined impurities from the natural gas, and a drying unit that dehydrates the natural gas treated with the removal device.

The transfer line can be equipped with a pipeline and valve.

The return line can be equipped with piping, LNG pump and automatic on / off valve.

A feed line may be provided that feeds the exhaust gas to the recondenser from the LNG tank. The supply line may be equipped with any one or more of a pipeline, an automatic on / off valve, a flow regulator, and a pressure regulator.

A pressure gauge may be provided that measures the pressure in the LNG tank. When the pressure of the pressure gauge reaches or exceeds a predetermined value, the supply and return fittings can be opened and the exhaust gas can be supplied to the recondenser through the supply line.

The condenser can be controlled to increase the cooling capacity of the recondenser when the pressure of the pressure gauge installed on the supply line reaches or exceeds a predetermined value. For example, control may be performed to increase the supplied amount of refrigerant, for example, supplied from a refrigeration unit (first or second refrigeration unit).

The LNG carrier may be, for example, a loading station container, a loading pier, a loading dock of a loading station, and the like.

A recovery line may be provided to return the exhaust gas in the LNG conveyor to the LNG tank.

In the third LNG production system

The refrigerant stored in the refrigerant buffer can be delivered from a refrigeration unit or an external refrigeration unit.

The condenser may have a piping that carries the refrigerant supplied from the refrigeration unit and the piping that carries the refrigerant supplied from the coolant buffer storage as separate components, and the return refrigerants can be returned to the refrigeration unit together.

The condenser may have a first heat exchanger to which the refrigerant supplied from the refrigeration plant is introduced, and a second heat exchanger to which the refrigerant supplied from the coolant buffer accumulator is introduced.

In each of the above-described first to third LNG production systems, the recondenser is preferably configured as follows.

The condenser is configured to recondense (liquefy) the stripping gas using a refrigerant at a pressure lower than the operating pressure of the LNG tank.

According to the configuration, the exhaust gas can be recondensed at a pressure lower than the operating pressure of the LNG tank without using a conventional exhaust gas compressor.

The condenser can be provided internally with a heat exchanger, inside which refrigerant is introduced, and the exhaust gas can be introduced inside the heat exchanger, and cooled by the refrigerant. As a result, the exhaust gas can be efficiently liquefied in heat exchanger mode.

The volume (external capacity) of the heat exchanger may be less than the internal volume (capacity of the internal space) of the re-condenser, and the heat exchanger may be located in the internal space of the re-condenser.

As a result, the exhaust gas can be efficiently liquefied in heat exchanger mode. Liquefied LNG is collected at the bottom of the condenser. The accumulated LNG can be fed into the LNG tank by a liquid pump.

The pressure in the condenser or in the heat exchanger can be controlled as follows.

(1) Before the exhaust gas is sent, refrigerant is supplied and pre-cooling is carried out inside the re-condenser or inside the heat exchanger. After a predetermined period of time has elapsed, or when a predetermined temperature is reached inside the recondenser or inside the heat exchanger, exhaust gas injection is started.

(2) The injected exhaust gas is liquefied and stored in the lower part of the condenser or heat exchanger. Liquefied LNG accumulating at the bottom can be supplied to the LNG tank by a pump, blower or the like.

The fourth LNG production system according to the present invention comprises

a liquefier that cools and liquefies natural gas using a refrigerant supplied from a refrigeration plant,

an LNG tank that stores liquefied natural gas, liquefied in a liquefier,

transfer line for transferring liquefied natural gas from the LNG tank,

an LNG transporter located at the next stage of the transfer line and designed to transport liquefied natural gas,

an LNG outlet line that removes liquefied natural gas from the LNG tank,

a subcooler, which is provided on the LNG outlet line and cools the liquefied natural gas using a refrigerant (for example, liquid nitrogen or the like),

a re-condenser that recondenses the stripping gas generated by the heat transferred to the LNG with the LNG cooled in a subcooler, and

a return line that feeds the liquefied natural gas that has been liquefied to the LNG tank from the recondenser.

In the present invention, the re-condenser can recondense (liquefy) the stripping gas with LNG cooled in a subcooler at a pressure lower than the operating pressure of the LNG tank.

According to the configuration, the liquefied natural gas is first cooled using a refrigerant such as LN ₂ , and the stripping gas is liquefied using the cooled liquefied natural gas. As a result, the boil-off gas recondensation can be efficiently performed at a pressure lower than the operating pressure of the LNG tank.

In a fourth aspect of the present invention, the subcooler can be controlled so that the liquefied natural gas is at a higher temperature than the crystallization point of the liquefied natural gas by a pressure regulator or a flow regulator installed in the refrigerant line through which the refrigerant flows.

In a fourth aspect of the present invention, two or more subcoolers may be employed. In the case of two subcoolers, the first recondensation process consists of liquefying the exhaust gas using the refrigerant supplied from the first subcooler, and the second recondensing process, which consists of liquefying the exhaust gas using the refrigerant supplied from the first subcooler and the refrigerant supplied from the second subcooler to treating more stripping gas than the amount of stripping gas being processed during the first recondensation process may be accomplished by switching between the first recondensation process and the second recondensing process. In case of switching from the first recondensation process to the second recondensing process, the operation of the refrigeration unit can be stopped or the refrigeration unit can operate continuously without stopping.

The condenser may comprise a switch control unit that switches between the first recondensation process and the second recondensation process.

The switch control unit can switch from the first recondensation process to the second recondensation process in case of transferring the boil-off gas to the LNG conveyor.

The changeover control unit can switch from the first recondenser process to the second recondenser process when the pressure value measured by a pressure gauge located in the LNG tank or on the supply line that supplies exhaust gas to the recondenser becomes equal to or exceeds a predetermined value.

The refrigerant in the second subcooler may be supplied from a refrigerant buffer storage in which the refrigerant is stored in advance.

In the above-described LNG production system, the pump for supplying liquefied natural gas (LNG) to the transfer line from the LNG tank may be a submersible pump installed inside the LNG tank, or may be a pump located on the transfer line.

[Brief description of graphic materials]

- In FIG. 1 is a diagram illustrating a configuration example of an LNG production system according to Embodiment 1.

- In FIG. 2 is a diagram illustrating a configuration example of an LNG production system according to Embodiment 2.

- In FIG. 3 is a diagram illustrating a configuration example of an LNG production system according to Embodiment 3.

- In FIG. 4A is a diagram illustrating a configuration example of a recondenser.

- In FIG. 4B is a diagram illustrating a configuration example of a recondenser.

- In FIG. 4C is a diagram illustrating a configuration example of a recondenser.

- In FIG. 5A is a diagram illustrating a configuration example of the LNG production system in Embodiment 4.

- In FIG. 5B is a diagram illustrating a configuration example of a recondenser.

- In FIG. 5C is a diagram illustrating a configuration example of a recondenser.

- In FIG. 6 is a diagram illustrating a configuration example of a conventional LNG production system.

[Description of Embodiments]

Hereinafter, some embodiments of the present invention will be described. The embodiments described below illustrate only examples of the present invention. The present invention is in no way limited by the following embodiments, and also provides various modified modes, performed within the scope and without changing the essence of the present invention. It should be noted that all of the components described below are not always necessary components of the present invention.

(Embodiment 1)

An LNG production system 1 according to Embodiment 1 will be described with reference to FIG. 1. The LNG production system 1 has a first line L1 for moving natural gas to a stage in the next stage, a compressor 11, and a second line L2 (eg a pipe). As a step in the next step, a predetermined substance (eg CO ₂ ) is removed, and the removal unit 12 is located there. Further, the SG after removal is fed to the drying unit 13 via the third line L3 and is subjected to a drying treatment. Next, the dried SG is fed to the liquefier 14 through the fourth line L4 and liquefied. A refrigerant (liquid refrigerant) is supplied to the liquefier 14 from the refrigeration unit 15 to cool the NG, and LNG is produced. Thereafter, the heat exchanged refrigerant is returned to the refrigeration unit 15 in a vaporized state. The LNG is fed to the LNG tank 16 via the fifth line L5 and stored. Lines 1 through 5 L1-L5 are equipped with pipes and on / off valves, for example. A predetermined control device (controller) controls the operation of the corresponding devices, the opening and closing of valves, the amount of LNG production, etc. of the LPG production system 1.

A first submersible pump P1 is disposed in the LNG tank 16, and the LNG in the tank is fed to the inside of the LNG conveyor 18 via a transfer line L6 by the first pump P1. As the LNG transporter 18, for example, a loading station container, a loading dock, a loading dock of a loading station, etc. are shown. The exhaust gas in the LNG conveyor 18 is fed to the LNG tank 16 via the recuperation line A2. Instead of or in addition to the recuperation line A2, a second supply line may be provided to supply the exhaust gas contained in the LNG conveyor 18 to the recondenser 17.

Exhaust gas is generated in the LNG tank 16 due to the heat input. In addition, exhaust gas is also generated when LNG is supplied from the liquefier 14. In addition, exhaust gas is also generated when LNG is supplied to the LNG carrier 18. Thus, the exhaust gas in the LNG tank 16 is supplied to the recondenser 17 via the first supply line A1. In addition, the exhaust gas in the transfer line L6 is supplied to the recondenser 17 via the third supply line A3.

The refrigerant (liquid refrigerant) is introduced into the recondenser 17 through the refrigerant line B1 from the refrigeration unit 15. With the help of the refrigerant, the exhaust gas supplied through each of the supply lines is recondensed (liquefied). The configuration of the recondenser 17 is described later. The LNG obtained from the recondensation (liquefaction) is returned to the LNG tank 16 through the return line A4. A second pump P2 is located on the return line A4 and LNG is supplied to the LNG tank 16 by driving the second pump P2.

According to the present embodiment, there is no need for successive stages in which exhaust gas is supplied to a drying unit and exhaust gas is supplied to an NGL liquefier to liquefy the exhaust gas as in the prior art. Consequently, it is not necessary to operate the entire LNG production system and only refrigeration unit 15 can be operated. Recondenser 17 can recondense the exhaust gas to LNG so that the liquefier 14's liquefaction capacity can be fully utilized to liquefy the GHG supplied from the dryer.

(Condenser)

FIG. 4A and FIG. 4B illustrates an embodiment of the recondenser 17. In FIG. 4A, the recondenser 17 has an outer wall 171 and a heat exchanger 172 enclosed by an outer wall 171. Refrigerant (liquid refrigerant) is introduced into the heat exchanger 172 from the refrigeration unit 15 through the refrigerant line B1, and the exhaust gas is cooled by taking energy from the cold refrigerant. The refrigerant is evaporated and returned to the refrigeration unit 15 through the refrigerant return line B2. LNG is supplied to the LNG tank 16 from the recondenser 17 via a second pump P2.

The condenser 17 is configured to recondense (liquefy) the exhaust gas using a refrigerant at a pressure lower than the operating pressure of the LNG tank 16.

The first supply line A1 can be equipped with a safety valve in case the pressure in the LNG tank 16 becomes abnormally high. In addition, an automatic on / off valve 42 is provided on the first supply line A1 for controlling the supply of exhaust gas to the re-condenser 17. In addition, a pressure gauge and a pressure regulator 41 are provided on the first supply line A1, which is controlled in accordance with the pressure gauge.

The operating pressure in the LNG tank 16 is on average 1.2 bar abs. (120 kPa abs.) Absolute pressure and is controlled within ± 15% of the upper and lower limits. When a large amount of exhaust gas is generated, the internal tank pressure becomes high. The internal pressure of the tank is measured with a pressure gauge, and based on the measurement result (conversion result), the valve control unit (not illustrated) controls the opening and closing of the automatic on-off valve 42. For example, when the internal pressure of the tank becomes 1.3 times the value of 1.2 bar abs. (120 kPa abs), the exhaust gas is supplied to the recondenser 17. With regard to the pressure regulator 41, the in-line pressure of the first supply line A1 is measured, and the opening degree of the regulator is controlled based on the measurement result.

The refrigerant supplied from the refrigeration unit 15 can be any medium with a lower temperature than the boiling point of the LNG, and can be used, for example, LN ₂ .

The internal pressure of the heat exchanger 172 is controlled to be less than the operating pressure (1.2 bar abs (120 kPa abs) average) of the LNG tank 16 during the exhaust gas recondensation process. The internal pressure of the heat exchanger 172 is measured with a pressure gauge and adjusted to be lower than the operating pressure of the LNG tank 16. In the present embodiment, the refrigerant contacts the exhaust gas in the heat exchanger 172, whereby the volume of the exhaust gas decreases due to liquefaction, and the pressure in the heat exchanger 172 decreases. During operation, the low pressure state is maintained with refrigerant supplied continuously. The internal pressure of the heat exchanger 172 is controlled by controlling the flow rate of the refrigerant. A flow controller (not illustrated) is provided on the refrigerant supply line B1, and the flow rate of the refrigerant can be controlled by the flow controller in accordance with the measurement of the pressure gauge measuring the internal pressure of the above-described heat exchanger 172.

It should be noted that the recondenser 17 is not limited to the mode of the heat exchanger 172, but there may be a mode in which the exhaust gas and the refrigerant are brought into direct contact with each other. As a method for contacting the exhaust gas and the refrigerant with each other, means for spraying the refrigerant using a sprinkler, means for bringing both of them into contact using a filler, and the like are given. The bottom of the heat exchanger 172 and the return line A4 are connected. The automatic on / off valve (not illustrated) provided on the return line A4 is controlled by opening and closing it and controlling the second pump P2, whereby LNG can be fed back to the LNG tank 16 from the recondenser 17.

Further in this document, the procedure for carrying out the technological process of boil-off gas recondensation (OG) will be described.

(1) The refrigerant is supplied to the heat exchanger 172 from the refrigeration unit 15, and the heat exchanger 172 is pre-cooled when the internal pressure of the LNG tank 16 exceeds the first threshold value. The temperature of the refrigerant is preferably set as a temperature that is, for example, above the crystallization point of the LNG and below the temperature of the LNG in the LNG tank 16. The temperature of the LNG that is cooled can be set based on the amount of the exhaust gas and the amount of the LNG that is cooled.

(2) When the heat exchanger 172 reaches a predetermined temperature or a temperature below it, the delivered amount of refrigerant is controlled by a flow controller (not illustrated) provided on the refrigerant line B1 to maintain the temperature.

(3) When the internal pressure of the LNG tank 16 exceeds the second threshold (the second threshold is greater than the first threshold), open the automatic on / off valve 42 and pressure regulator 41 and inject exhaust gas directly into the heat exchanger 172 of the recondenser 17 from the LNG tank 16. During the introduction of the exhaust gas, the amount of refrigerant delivered to the heat exchanger 172 is controlled and a negative pressure (or a pressure lower than the operating pressure of the LNG tank 16) is maintained within the heat exchanger 172.

(4) The heat exchanger 172 is pre-cooled and the exhaust gas is immediately cooled and changes state to LNG and the LNG is lowered to the bottom of the heat exchanger 172.

(5) LNG is fed back to the LNG tank 16 via return line A4.

(6) Close the appropriate fittings after the end of the recondensation process.

It is assumed that state (3) (the internal pressure of the tank exceeds the second threshold value) is established during the technological processes (1) and / or (2), so that the exhaust gas can be adapted to be released from the LNG tank 16 using a safety valve (not illustrated), or the exhaust gas can be discharged into the outside air by ventilation, which is not illustrated.

The above-described "first threshold value" is, for example, a pressure that is 1.26 times the value of 1.2 bar abs. (120 kPa abs).

The above-described "second threshold value" is, for example, a pressure that is 1.3 times the value of 1.2 bar abs. (120 kPa abs).

A pressure regulator (not illustrated) or a flow regulator (not illustrated) can be installed in the refrigerant line B1, and the refrigerant supply (VN) and the exhaust gas supply (VB) can be controlled so that VN exceeds VB.

(Other options for implementation)

The recondenser of FIG. 4B. FIG. 4B, in the recondenser 17, the volume (external capacity) of the heat exchanger 172 is smaller than the internal volume (internal space) of the recondenser 17, and the heat exchanger 172 is located in the internal space 173 of the recondenser. In heat exchanger mode, the exhaust gas can be efficiently liquefied. The liquefied LNG is collected in the lower part of the inner space 173 of the recondenser 17. The accumulated LNG can be fed to the LNG tank 16 by a second pump P2.

The upper part (preferably the upper side of the heat exchanger 172) of the inner space 173 of the recondenser 17 and the first supply line A1 are directly connected. In addition, the lower part of the inner space 173 of the recondenser 17 and the return line A4 are directly connected.

The internal pressure of the recondenser 17 is controlled to be a pressure lower than the operating pressure (1.2 bar abs. (120 kPa abs.) Average) of the LNG tank 16 during the exhaust gas recondensation process. The internal pressure of the re-condenser 17 is measured with a pressure gauge and adjusted to be lower than the operating pressure of the LNG tank 16.

In the present embodiment, the refrigerant is supplied to the heat exchanger 172, and as a result, cooling occurs inside the recondenser 17. When the exhaust gas is introduced into the cooled recondenser 17, the volume of the exhaust gas decreases due to liquefaction, and the pressure inside the recondenser 17 decreases. During operation, refrigerant is continuously supplied to heat exchanger 172 and therefore continues to cool inside the exhaust gas liquefaction re-condenser 17 to maintain a low pressure state inside the re-condenser 17. The internal pressure of the re-condenser 17 is controlled by controlling the refrigerant flow. A flow controller is provided on the refrigerant line B1, and the flow rate of the refrigerant can be controlled by the flow controller in accordance with the measurement result of the pressure gauge that measures the internal pressure of the above-described re-condenser 17, and the refrigerant flow can be controlled by controlling the opening degree of the automatic on / off valve provided on the refrigerant line B1, or you can control both of them.

Further in this document, the procedure for carrying out the technological process of boil-off gas recondensation (OG) will be described.

(1) The refrigerant is supplied to the heat exchanger 172 and the pre-cooling of the recondenser 17 is performed when the internal pressure of the LNG tank 16 exceeds the first threshold value. The temperature of the refrigerant is preferably set as a temperature that is, for example, above the crystallization point of the LNG and below the temperature of the LNG in the LNG tank 16. The temperature of the LNG that is cooled can be set based on the amount of the exhaust gas and the amount of the LNG that is cooled.

(2) When the re-condenser 17 reaches a predetermined temperature or a temperature below it, the delivered amount of refrigerant is controlled by a flow controller (not illustrated) provided on the refrigerant line B1 to maintain the temperature.

(3) When the internal pressure of the LNG tank 16 exceeds the second threshold value (the second threshold value exceeds the first threshold value), the automatic on-off valve 42 and the pressure regulator 41 are opened, and exhaust gas is introduced into the recondenser 17 from the LNG tank 16. During the introduction of the exhaust gas, the amount of refrigerant delivered to the heat exchanger 172 is controlled and a negative pressure (or a pressure lower than the operating pressure of the LNG tank 16) is maintained within the heat exchanger 172.

(4) The condenser 17 is pre-cooled and the exhaust gas is immediately cooled and changes state to LNG and the LNG is accumulated in the bottom of the recondenser 17.

(5) The LNG that accumulates at the bottom of the recondenser 17 is fed back to the LNG tank 16 through the return line A4.

(6) Close the appropriate fittings after the end of the recondensation process.

It is assumed that state (3) (the internal pressure of the tank exceeds the second threshold value) is established during the technological processes (1) and / or (2), so that the exhaust gas can be adapted to be released from the LNG tank 16 using a safety valve (not illustrated), or the exhaust gas can be discharged into the outside air by ventilation, which is not illustrated.

The above-described "first threshold value" is, for example, a pressure that is 1.26 times the value of 1.2 bar abs. (120 kPa abs).

The above-described "second threshold value" is, for example, a pressure that is 1.3 times the value of 1.2 bar abs. (120 kPa abs).

A pressure regulator (not illustrated) or a flow regulator (not illustrated) can be installed in the refrigerant line B1, and the refrigerant supply (VN) and the exhaust gas supply (VB) can be controlled so that VN exceeds VB.

[0043]

(Embodiment 2)

The LNG production system 2 according to Embodiment 2 will be described using FIG. 2. Components with the same reference numerals as in the LNG production system 1 according to Embodiment 1 have the same functions, and therefore, explanation regarding the components will be omitted or brief.

The LNG production system 2 according to embodiment 2 comprises a first refrigeration unit 15 and a second refrigeration unit 20. The first refrigeration unit supplies refrigerant to the refrigeration unit 14. The second refrigeration unit 20 supplies refrigerant (liquid refrigerant) to the recondenser 17 via refrigerant line C1 (corresponding to position B1 in Fig. 1) and returns the refrigerant used as a source of cold to the recondenser 17 along the return line C2 (corresponding to position B2 in Fig. 1).

Therefore, since the second refrigeration unit 20 is provided separately from the first refrigeration unit 15, it is not necessary to deliver the refrigerant to the operating refrigeration unit 14 from the large refrigeration unit and to deliver the refrigerant to the re-condenser 17, it is not necessary to install the refrigeration unit larger than necessary, and can be installed only a medium or small refrigeration unit, so that the installation space can be small, and the initial cost and operating cost can be reduced.

(Embodiment 3)

The LNG production system 3 according to Embodiment 3 will be described with reference to FIG. 3. Components with the same reference numerals as in the LNG production system 1 of Embodiment 1 have the same functions, and therefore, explanation regarding the components will be omitted or brief.

The condenser 17 of the LNG production system 3 according to Embodiment 3 may carry out a first recondensation process of liquefying exhaust gas with a refrigerant supplied from a refrigeration unit 15, and a second recondensing process in which an exhaust gas is liquefied using a refrigerant supplied from a refrigeration unit 15 and the refrigerant supplied from the coolant buffer storage 30 to treat more exhaust gas than the amount of exhaust gas processed during the first recondensation process by switching between the first and second recondensing processes.

The refrigerant buffer storage 30 is supplied with refrigerant from the refrigeration unit 15 through the first delivery line E1 and / or the refrigerant is delivered from an external refrigerant source through the second delivery line E2 and stored in advance. During operation of the recondenser 17, the refrigerant is introduced into the recondenser 17 via the buffer accumulator line D1 from the refrigerant buffer accumulator 30.

The condenser 17 includes a switch control unit (not illustrated) that switches between the first recondensation process and the second recondensation process.

When the exhaust gas moves to the LNG conveyor 18, the switching control unit can switch from the first recondensation process to the second recondensing process in response to the moment of the start of the movement, which is planned, or the moment when the determining unit determines, for example, that LNG is moving from the tank 16 for LNG. As a determination unit, a determination unit is given that determines that a transport vessel enters a harbor, a determination unit that determines that the automatic on / off valve of the travel line L6 opens, a determination unit using a control signal to control the automatic on / off valve as a determination signal, a determination unit, determining that the measurement result of the flow meter located on the travel line L6 reaches or exceeds a threshold value, and the like.

In addition, the changeover control unit can switch from the first recondensation process to the second recondensation process when the pressure value inside the LNG tank 16 measured by the pressure gauge or the pressure value measured by the pressure gauge located on at least any of the supply line A1, recuperation line A2 and supply line A3 reaches or exceeds a predetermined value.

An example of a recondenser 17 according to Embodiment 3 will be described with reference to FIG. 4C. The condenser 17 has a first heat exchanger 172 and a second heat exchanger 174 in its inner space 173. Refrigerant is introduced into the first heat exchanger 172 from the refrigeration unit 15 during the first recondensation process and cools the exhaust gas. When the generated amount of exhaust gas is large, the changeover control unit switches from the first recondensation process to the second recondensation process. During the operation of the first heat exchanger 172, the second heat exchanger 174 also operates. The refrigerant is introduced into the second heat exchanger 174 through the buffer tank line D1 from the coolant buffer tank 30. As a result, cooling is carried out by means of two heat exchangers, so that during the peak load period (for example, in the case of processing a large amount of exhaust gas generated, for example, when LNG is supplied to the LNG transport vessel), during which the amount of exhaust gas that is supplied is higher, than the amount of exhaust gas at normal times, the exhaust gas is also efficiently cooled to be converted to LNG and the LNG can be returned to the LNG tank 16. It should be noted that the refrigerant used in the second heat exchanger 174 is adapted to enter the refrigerant return line B2 of the first heat exchanger 172 through the refrigerant return line D2, but the present invention is not limited to this, and the refrigerant return line D2 may be connected to the refrigeration unit 15.

In addition, when the exhaust gas is supplied to the LNG conveyor 18, the changeover control unit can switch from the second recondensation process to the first recondensation process in response to the end time of the movement that is scheduled, or the point in time when the determination unit determines, for example, that the movement LNG from LNG tank 16 completed. As a determination unit, a determination unit is given that determines that the automatic on-off valve on the line L6 of travel is closed, a determination unit that determines a control signal that controls the automatic on-off valve, as a determination signal, a determination unit that determines that the measurement result of the flow meter located on the movement line L6 becomes equal to or less than the threshold value, and the like.

In addition, the changeover control unit can switch from the second recondensation process to the first recondensation process when the pressure value inside the LNG tank 16 measured by the pressure gauge or the pressure value measured by the pressure gauge located in at least one of the supply line A1, recuperation line A2 and supply line A3 becomes less than a predetermined value.

(Other options for implementation)

In the above described embodiments, two heat exchangers are disposed in the recondenser 17, and the cooling capacities of the heat exchangers may be the same or different.

In the present embodiment, the combination of the buffer storage tank and the heat exchanger is provided in a single amount, but the present invention is not limited thereto, and two or more combinations may be provided.

(Embodiment 4)

The LNG production system 5 in Embodiment 4 will be described with reference to FIG. 5A and FIG. 5B. Components with the same reference numerals as in the LNG production system 1 according to Embodiment 1 have the same functions, and therefore, explanation regarding the components will be omitted or brief.

In Embodiment 4, the refrigeration unit is not used in the LNG production system, but LNG is used in the LNG tank. That is, in Embodiment 4, LNG is subcooled to a predetermined temperature with a refrigerant, and the LNG is supplied to a recondenser to be contacted with the exhaust gas to liquefy the exhaust gas.

Embodiment 4 comprises a first feed line A1 that feeds exhaust gas from the LNG tank 16, an LNG outlet line E1 that discharges LNG from the LNG tank 16, a subcooler 52 that cools the LNG with a refrigerant, a condenser 57 that liquefies the exhaust gas supplied through the first exhaust gas supply line A1, with the help of LNG cooled in the subcooler 52, at a pressure lower than the operating pressure of the LNG tank 16, and the return line A4, which returns the LNG, that is, the exhaust gas liquefied in the recondenser 57, to tank 16 for LNG. The respective components will be described in detail later in this document.

The first supply line A1 may be equipped with a safety valve (not illustrated) in case the pressure in the LNG tank 16 becomes abnormally high. In addition, the first supply line A1 is equipped with an automatic on / off valve 42 and a pressure regulator 41 for controlling the supply of exhaust gas to the condenser 10.

The operating pressure in the LNG tank 16 is on average 1.2 bar abs. (120 kPa abs.) Absolute pressure and is controlled within ± 15% of the upper and lower limits. When a large amount of exhaust gas is generated, the internal tank pressure becomes high. The internal pressure of the reservoir is measured with a pressure gauge, and a valve control unit (not illustrated) controls the opening and closing of the automatic on / off valve 42 based on its measurement result (conversion result). For example, when the internal pressure of the tank becomes 1.3 times higher than 1.2 bar abs. (120 kPa abs), the exhaust gas is supplied to the recondenser 57. The pressure regulator 41 measures the internal pipe pressure of the first supply line A1 and controls the valve opening degree based on the measurement result.

LNG is introduced into the subcooler 52 via the LNG outlet line E1 from the LNG tank 16. The valve control unit (not illustrated) controls the opening and closing of the automatic on / off valve 51 provided on the LNG outlet line E1 and controls the liquid pump P5, whereby LNG is supplied to the subcooler 52 from the LNG tank 16 and fed to the condenser 57 at the next steps. The submersible liquid pump P1 may be adapted to supply LNG instead of the liquid pump P5.

The refrigerant in the subcooler 52 may be below the boiling point of LNG, and LN ₂ is used in the present embodiment. LN ₂ is introduced into the subcooler 52 through refrigerant line F1 from a source LN ₂ (for example a reservoir for LN2) and is used as a cold source to cool the LNG flowing inside the subcooler 52. LN ₂ can be converted to gaseous form or can be vented in the form of a fluid in which liquid and gas are mixed when discharging LN ₂ from the subcooler 52 through the outlet line F2. The fluid (LN ₂ and / or GN ₂ ) that is discharged may be treated to purify the discharged substances to the atmosphere or may be subjected to a recycling process.

The LNG subcooler 52 can be controlled to be above the crystallization point of the LNG by a pressure regulator (not illustrated) or a flow regulator (not illustrated) installed on the refrigerant line F1 through which refrigerant flows (LN ₂ ).

The internal pressure of the recondenser 57 is controlled to be a pressure lower than the operating pressure (1.2 bar abs (120 kPa abs) average) of the LNG tank 16 during the exhaust gas recondensation process. The internal pressure of the recondenser 57 is measured with a pressure gauge and adjusted to be lower than the operating pressure of the LNG tank 16.

In the present embodiment, the LNG cooled in the subcooler 52 contacts the exhaust gas in the re-condenser 57, whereby the amount of the exhaust gas decreases due to liquefaction, and the pressure in the re-condenser 57 decreases. During operation, the low pressure state is maintained with chilled LNG delivered continuously. The internal pressure of the recondenser 57 is controlled by controlling the flow rate of the cooled LNG. On the LNG supply line E2, a flow regulator is provided between the subcooler 52 and the recondenser 57, and the flow rate of the LNG can be controlled by the flow regulator in accordance with the measurement of the pressure gauge that measures the internal pressure of the above-described recondenser 57, while the LNG flow rate can be controlled by controlling the opening degree of the automatic on-off valve 51, or both of them can be controlled.

The exhaust gas introduced into the recondenser 57 is brought into contact with the cooled LNG, whereby the exhaust gas is liquefied to become LNG, and the LNG is accumulated in the lower part of the recondenser 57. As a method of bringing both of the exhaust gas and the LNG into contact with each other, atomizing means are provided with the LNG sprinkler cooled in the subcooler 52, means for bringing both of them into contact with each other using filler, and the like.

The lower part of the recondenser 57 and the return line A4 are connected. The valve control assembly (not illustrated) performs on / off control of the automatic on / off valve 54 provided on return line A4 and controls the liquid pump P2, whereby the valve control assembly can feed LNG back to the LNG tank 16 from the recondenser 57.

Further in this document, the procedure for carrying out the technological process of exhaust gas recondensation will be described. Corresponding valves 41-42, 51 and 54 are in a closed state, except in the process of recondensing.

(1) A refrigerant (LN ₂ , for example) is supplied to the subcooler 52 when the internal pressure of the LNG tank 16 exceeds the first threshold.

(2) When the subcooler 52 reaches a predetermined temperature or a temperature below it, LNG is supplied to the subcooler 52 from the LNG tank 16 and cooled. For example, the temperature of the LNG that is cooled is preferably set as a temperature that is above the crystallization point of the LNG and below the temperature of the LNG in the LNG tank 16. The temperature of the LNG that is cooled can be set based on the amount of the exhaust gas and the amount of the LNG that is cooled.

(3) The cooled LNG is supplied to the re-condenser 57 and the pre-cooling of the re-condenser 57 is performed. The automatic on-off valve 54 of the return line A4 is closed.

(4) When the internal pressure of the LNG tank 16 exceeds the second threshold (the second threshold is greater than the first threshold), the automatic on-off valve 42 and the pressure regulator 41 are opened and exhaust gas is introduced into the recondenser 57 from the LNG tank 16.

(5) The condenser 57 is pre-cooled and the cooled LNG is introduced into the recondenser 57 together with the exhaust gas, whereby the exhaust gas is cooled and changes state to CNG, and the LNG is accumulated in the lower part of the recondenser 57.

(6) When the LNG that accumulates in the lower part of the recondenser 57 reaches a predetermined amount, (or at a predetermined time), the automatic on-off valve 54 is opened, the liquid supply pump P2 is controlled, and LNG is supplied to the LNG tank 16 from the recondenser 57.

(7) Close the appropriate fittings after completion of the recondensation process.

It is also assumed that state (4) (tank internal pressure exceeds the second threshold) is set during processes (1) to (3), so that the exhaust gas can be adapted to be discharged from the LNG tank 16 using a safety valve (not illustrated ), or the exhaust gas can be discharged into the outside air by ventilation, which is not illustrated.

The above-described "first threshold value" is, for example, a pressure that is 1.26 times the value of 1.2 bar abs. (120 kPa abs).

The above-described "second threshold value" is, for example, a pressure that is 1.3 times the value of 1.2 bar abs. (120 kPa abs).

(Embodiment 5)

An LNG production system according to Embodiment 5 will be described using FIG. 5C. Components with the same reference numerals as in the LNG production systems 1 and 5 of Embodiments 1 and 4 have the same functions, and therefore, explanation regarding the components will be omitted or brief.

In embodiment 5, there are first and second subcoolers, and there is a switch between the first recondensation process, which consists in liquefying the exhaust gas with the refrigerant supplied from the first subcooler 52, and the second recondensing process, which consists in liquefying the exhaust gas with the refrigerant supplied from the first a subcooler 52, and a refrigerant supplied from the second subcooler 521 to treat more exhaust gas than the amount of exhaust gas processed during the first recondensation process. According to an embodiment, if the process is carried out over the exhaust gas in an amount that is in a predetermined range (flow rate per unit time), or if the pressure value is within a predetermined range, set in advance, the first recondensing process is carried out (the exhaust gas liquefaction process with with LNG cooled in the first subcooler), and if the process is carried out over the exhaust gas in an amount exceeding the amount that is in a predetermined range, or if the pressure value is in the predetermined range described above, a second process of recondensation can be carried out (with the simultaneous execution of the exhaust gas liquefaction process using LNG cooled in the second subcooler, while the process of exhaust gas liquefaction using LNG cooled in the first subcooler continues).

The changeover control unit (not illustrated) can switch from the first recondensation process to the second recondensation process in the event of exhaust gas transfer to the LNG conveyor, or can switch from the first recondensation process to the second recondensation process when the pressure value measured by a pressure gauge located in the LNG tank or on the supply line A1, which supplies exhaust gas to the recondenser 57, reaches or exceeds a predetermined value.

In the present embodiment, the refrigerant supplied to the first subcooler 52 and the refrigerant supplied to the second subcooler 521 may be the same refrigerants or may be different refrigerants. For example, a mixture such as hydrocarbon may be specified as the refrigerant in the first subcooler 52, and nitrogen or the like may be specified as the refrigerant in the second subcooler 521.

The switch control unit may switch from the first recondensation workflow to the second recondensation workflow at a point in time according to Embodiment 3 described above. When the process is switched to the second recondenser process, a valve control unit (not illustrated) controls the opening and closing of valve 53, supplies LNG to the second subcooler 521, and supplies LNG to the recondenser 57 in the next stage. That is, in the first recondensation process, cooled LNG is supplied to the recondenser 57 via the LNG supply line E2 from the first subcooler 52, but the process is switched to the second recondensing process, and the cooled LNG is supplied to the recondenser 57 via the LNG supply line E21 from the second subcooler 521 in addition to the chilled LNG supplied to the recondenser 57 from the first subcooler 52.

The refrigerant of the second subcooler 521 can be any medium below the boiling point of LNG, and LN ₂ is used in the present embodiment. LN ₂ is introduced into the second subcooler 521 via refrigerant line F11 from a source LN ₂ (e.g. a reservoir for LN ₂ ) and is used as a cold source to cool the LNG flowing inside the second subcooler 521. LN ₂ can be converted to gaseous form or can be discharged as a fluid in which liquid and gas are mixed when discharging LN ₂ through discharge line F21 from the second subcooler 521. The fluid (LN2 and / or GN2) that is discharged can be treated to purify discharged substances in atmosphere or may be subjected to a recycling process. In addition, the second LNG subcooler 521 can be controlled to a temperature that is above the crystallization point of the LNG by a pressure regulator (not illustrated) or a flow regulator (not illustrated) installed on the refrigerant line F11 through which the refrigerant flows ( LN ₂ ).

(Other options for implementation)

In Embodiments 4 and 5 described above, corresponding automatic on / off valves, pressure regulators and fluid pumps are provided on respective lines, but some or all of them may be omitted in accordance with the intended use without being limited to the above provisions.

Further, in Embodiments 4 and 5 described above, in the exhaust gas recondensation process, LNG that is cooled is supplied to pre-cool the recondenser 57 before feeding the exhaust gas to the recondenser 57, but the present invention is not limited to this, and the cooled LNG and exhaust gas may be served together. In this case, the supply amount (VL) of the cooled LNG and the supply amount (VB) of the exhaust gas can be controlled so that VL is greater than VB. On the respective line E1, the LNG injection line E2 and the first exhaust gas supply line A1, flow controllers can be provided to control the flow rate of the respective supplied quantities.

In addition, in Embodiments 4 and 5 described above, a liquid supply pump P2 is provided on the return line A4, but a configuration in which a liquid supply pump is not provided on the return line A4 may be used. The LNG changed state from the exhaust gas in the recondenser 57 can be fed to the interior of the LNG tank 16 by gravity.

In addition, the refrigerant in the aforementioned second subcooler 521 can be supplied from the refrigerant buffer storage in which the refrigerant is stored in advance.

In each of all the above-described embodiments, the pump P1 is of submersible type, but the pump P1 is not limited thereto, and the pump P1 may be a pump located on the line L6 of movement.

[List of Reference Items]

1 LNG production system

14 Cooling device

15 Refrigeration unit

16 LNG tank

17 Condenser

18 LNG transporter

L6 Travel line

Claims (24)

1. System for the production of liquefied natural gas (LNG), containing: a liquefier that cools and liquefies natural gas using a refrigerant supplied from a refrigeration unit; an LNG tank that stores liquefied natural gas liquefied in a liquefier; transfer line for transferring liquefied natural gas from the LNG tank; an LNG transporter located at the next stage of the transfer line and designed to move liquefied natural gas; a recondenser that is switched to alternately perform a first recondensation process of liquefying the boil-off gas generated by heat transferred to the liquefied natural gas with a refrigerant supplied from a refrigeration unit, and a second recondensation process of liquefying the boil-off gas using refrigerant supplied from the refrigeration unit and refrigerant supplied from the refrigerant buffer storage to treat a larger amount of stripping gas than the amount of stripping gas processed during the first recondensation process; and a return line that feeds the liquefied natural gas that has been liquefied to the LNG tank from the recondenser. 2. LNG production system according to claim 1, characterized in that the recondenser is configured to recondense the stripping gas using a refrigerant at a pressure lower than the operating pressure of the LNG tank. 3. LNG production system according to claim 1 or 2, characterized in that the recondenser is provided with a heat exchanger inside, into which a refrigerant is introduced, and a stripping gas is introduced into the inside of the heat exchanger and cooled by means of a refrigerant. 4. LNG production system according to claim 3, characterized in that the volume of the heat exchanger is less than the internal volume of the re-condenser, and the heat exchanger is located in the internal space of the re-condenser. 5. LNG production system comprising: a liquefier that cools and liquefies natural gas using a refrigerant supplied from a refrigeration unit; an LNG tank that stores liquefied natural gas liquefied in a liquefier; transfer line for transferring liquefied natural gas from the LNG tank; an LNG transporter located at the next stage of the transfer line and designed to move liquefied natural gas; an LNG outlet line that removes liquefied natural gas from the LNG tank; a subcooler, which is provided on the LNG outlet line and cools the liquefied natural gas using a refrigerant; a re-condenser that recondenses the stripping gas generated by the heat transferred to the liquefied natural gas with the liquefied natural gas cooled in a subcooler; and a return line that feeds the liquefied natural gas that has been liquefied to the LNG tank from the recondenser. 6. LNG production system according to claim 5, characterized in that the recondenser is configured to recondense the stripping gas using a refrigerant at a pressure lower than the operating pressure of the LNG tank.

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