JP2013155879A - Method for reliquefying boil-off gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for reliquefying boil-off gas capable of effectively achieving reliquefaction, by boosting gaseous boil-off gas and bringing it into direct contact with delivered low-temperature liquid such as LNG to attain reliquefaction.SOLUTION: In a method for reliquefying boil-off gas, boil-off gas generated from low-temperature liquid 5 stored in a storage tank 3 is compressed by a boil-off gas compressor 7 and is introduced into a micro bubble generator 11, the low-temperature liquid 5 in the storage tank 3 is delivered by a delivery pump 9 and is introduced into the micro bubble generator 11. In the micro bubble generator 11, the boil-off gas is mixed with the low-temperature liquid to reliquefy the boil-off gas, and the low-temperature liquid containing the reliquefied boil-off gas is boosted by a secondary pump 13. The boil-off gas introduced into the micro bubble generator 11 is turned into micro bubbles, and the micro bubbles of the boil-off gas are cooled to be reliquefied in the micro bubble generator 11 and in a pipe connecting the micro bubble generator 11 with the secondary pump 13, by the low-temperature liquid.

Description

  The present invention is an evaporation that reliquefies evaporation gas ("boil-off gas", "BOG") generated in a storage tank in which a low-temperature liquid such as liquefied natural gas (hereinafter sometimes referred to as "LNG") is stored. The present invention relates to a gas reliquefaction method.

When low temperature liquid such as liquefied natural gas (LNG) is stored in a tank, a part of the low temperature liquid in the tank evaporates due to heat input to the tank, and evaporative gas is generated. When the low-temperature liquid is LNG, an evaporating gas mainly composed of methane is generated.
The generated evaporative gas can be compressed as it is and supplied as city gas, but the compression power becomes very large. Therefore, in order to reduce power, it is conceivable to re-liquefy the vaporized gas, pressurize it in a liquid state, and then gasify it again to supply it as city gas. In order to reliquefy, the evaporative gas is compressed and cooled, but the evaporative gas is cooled and cooled by the cold heat of the LNG, that is, an example in which heat exchange is performed between the LNG and the compressed evaporative gas. It is proposed in Patent Documents 1 to 3.

In patent document 1, the system (indirect heat exchange system) which heat-exchanges LNG and the compressed vapor gas with a heat exchanger is disclosed. However, the indirect heat exchange method has a problem that a large heat exchanger is required to secure a heat transfer area.
Patent Document 2 discloses a method (direct contact heat exchange method) in which compressed evaporative gas is directly blown into the discharge LNG. In the direct contact heat exchange system, heat exchange performance is improved at the interface where both are in contact with each other because heat exchange is performed without going through the heat transfer surface. However, in the direct contact heat exchange method, it is easy to separate into two phases due to the difference in density between gas evaporative gas and liquid LNG, and the liquefaction efficiency tends to deteriorate because the interface area (heat transfer area) cannot be sufficiently secured. . Therefore, it is important how to secure the interface area between the gaseous evaporative gas and the liquid LNG, but Patent Document 2 does not disclose anything about this point.
In Patent Document 3, as a method of injecting evaporative gas into LNG, a method in which an evaporative gas supply nozzle is arranged so as to be in a direction orthogonal to the flow direction of discharged LNG or in a direction opposite to the flow direction of LNG is disclosed. Has been.

JP S55-145897 JP H9-59657 JP H8-173781

However, even with the method of Patent Document 3, it is difficult to say that the interface area (heat transfer area) between the gaseous evaporative gas and the liquid LNG can be sufficiently secured.
As described above, the method of increasing the pressure of the gas evaporative gas and bringing it into direct contact with the discharged LNG is efficient, but it is important how to ensure a sufficient interface area between the two. If the interface area is not sufficiently secured, the evaporative gas that has joined the LNG will not be completely liquefied, and if it is supplied to the liquid feed pump without being liquefied, not only will the liquid feed be difficult, but the liquid feed pump will fail. It becomes the cause of.
As described above, in the method in which the evaporated gas is directly brought into contact with the LNG to be dispensed, it is important to ensure a sufficient interface area between the two. However, the development of such technology has been desired.

  The present invention has been made in order to solve such problems, and effectively re-liquefaction is achieved by increasing the pressure of the gas evaporative gas and bringing it into direct contact with a low-temperature liquid such as LNG to be discharged to re-liquefy. The object is to obtain an evaporative gas reliquefaction method.

(1) In the evaporative gas reliquefaction method according to the present invention, the evaporative gas generated from the low temperature liquid stored in the storage tank is compressed by the evaporative gas compressor and introduced into the fine bubble generator, and the low temperature in the storage tank Liquid is delivered by a delivery pump and introduced into the fine bubble generator. In the fine bubble generator, the evaporative gas and the low-temperature liquid are mixed to re-liquefy the evaporative gas, and the re-liquefied evaporative gas is discharged. An evaporative gas reliquefaction method for boosting a low-temperature liquid containing by a secondary pump,
The evaporative gas introduced into the microbubble generator becomes microbubbles, and the evaporative gas that has become microbubbles is generated by the cryogenic liquid in the microbubble generator and in the pipe connecting the microbubble generator and the secondary pump. It is cooled and reliquefied.

(2) In the device described in (1) above, the evaporative gas and the low-temperature liquid are boosted according to a mass flow rate ratio between the evaporative gas sent to the evaporative gas compressor and the low-temperature liquid sent from the storage tank. The pressure to control is controlled.

  In the present invention, the evaporative gas generated from the low-temperature liquid stored in the storage tank is compressed by the evaporative gas compressor and introduced into the fine bubble generator, and the low-temperature liquid in the storage tank is sent out by the delivery pump and finely discharged. Introduced into the bubble generator, the evaporative gas and the low-temperature liquid are mixed in the microbubble generator to make the evaporative gas into microbubbles, and the evaporated gas that has become microbubbles is contained in the microbubble generator and the microbubble generator. It can be cooled and reliquefied by the low-temperature liquid in the pipe connecting the secondary pump, and it can be prevented that the evaporative gas flows into the secondary pump that pressurizes the low-temperature liquid installed on the downstream side to cause a failure. .

Furthermore, by controlling the pressure at which the vaporized gas and the low-temperature liquid are boosted according to the mass flow rate ratio between the vaporized gas and the low-temperature liquid, the following effects can be obtained.
When the ratio of the evaporative gas is small, it is possible to sufficiently receive the cold heat from the low-temperature liquid, so that the evaporative gas can be reliably liquefied even if the pressure to be increased is lowered. By reducing the pressure to be increased, the power consumption of the evaporative gas compressor can be reduced.
On the other hand, when the proportion of the evaporative gas is large, it is not possible to sufficiently receive the cold heat from the low-temperature liquid. Therefore, the vapor can be reliquefied with a small amount of cold heat by increasing the vapor pressure accordingly, and the proportion of the vaporized gas is large. Even in cases, it can be reliquefied reliably.

It is explanatory drawing of the evaporative gas reliquefaction apparatus used for one embodiment of this invention. It is explanatory drawing of the one aspect | mode of the fine bubble generator used for the evaporative gas reliquefaction apparatus shown in FIG. It is explanatory drawing of the evaporative gas reliquefaction apparatus used for other embodiment of this invention. It is explanatory drawing of the control method of the evaporative gas compressor by the control apparatus of other embodiment shown in FIG.

[Embodiment 1]
An embodiment of the present invention will be described with reference to FIG.
An evaporative gas reliquefaction apparatus 1 used in an evaporative gas reliquefaction method according to the present invention includes an evaporative gas compressor 7 that compresses evaporative gas generated from a low temperature liquid 5 (for example, LNG) stored in a storage tank 3, and The evaporative gas compressed by the evaporative gas compressor 7 and the cryogenic liquid sent from the storage tank 3 by the primary pump 9 (corresponding to the “delivery pump” of the present invention) are mixed and mixed into the cryogenic liquid 5. And a fine bubble generator 11 for generating fine bubbles of evaporating gas. A secondary pump 13 is provided downstream of the microbubble generator 11 to pressurize the low-temperature liquid obtained by re-liquefying the evaporative gas, and a vaporizer 15 for vaporizing the low-temperature liquid is further provided downstream thereof. .
The main equipment which comprises the evaporative gas reliquefaction apparatus 1 is demonstrated in detail.

<Evaporative gas compressor>
The evaporative gas compressor 7 raises the evaporative gas to a pressure between the atmospheric pressure and the city gas operating pressure (4 to 7 MPa) (sometimes referred to as “intermediate pressure”). However, since the evaporative gas pressure needs to be the same level as that of the low temperature liquid introduced into the fine bubble generator 11, it is generally set to 0.5 MPa or more.

<Microbubble generator>
The fine bubble generator 11 mixes a low temperature liquid and evaporating gas to generate fine bubbles of evaporating gas in the low temperature liquid. The fine bubbles usually mean bubbles of 100 μm or less. As a method for generating fine bubbles, a known method such as a swirling liquid flow method, a static mixer method, a pore method, an ultrasonic method, or a venturi method (shock wave method) can be used. Among these, FIG. 1 shows a swirling flow type fine bubble generator 11. As a specific example of the swirling flow type fine bubble generator 11, there is a fine bubble generator 110 disclosed in, for example, Japanese Patent Application Laid-Open No. 2007-69071 shown in FIG. 2, and the same method is used in this embodiment as well. Can be used.

An outline of the fine bubble generator 11 is as follows based on FIG.
As shown in FIG. 2, the microbubble generator 110 includes a mixing container 113 in which a conical gas-liquid mixing space 111 is formed, and the mixing container 113 has a center on the bottom surface portion 115 on the large-diameter side. Is connected to a gas introduction pipe 117 that guides the evaporated gas to the space 111, and a liquid introduction pipe 121 is connected to the side surface portion 119 in a tangential direction. A discharge pipe 123 that discharges the gas-liquid mixture is connected to the front end side of the space 111.

  In the fine bubble generator 11 configured as described above, when the pressurized liquid is introduced from the liquid introduction pipe 121 into the space 111, the pressurized liquid is along the tangential direction of the circular cross section of the conical space 111. Flowing. Therefore, a swirl flow is generated in the space 111, and the conical space 111 becomes negative pressure due to the swirl flow. Thereby, the gas in the gas introduction pipe 117 is sucked into the space 111 along the central axis of the swirling flow from the bottom surface portion 115, and as a result, an air column 125 is generated as shown in FIG. On the other hand, in the air column 125, in the vicinity of the apex of the cone or in the vicinity of the discharge pipe 123, the swirl is suddenly weakened by the external stationary fluid, and the air column 125 is subjected to a uniform shearing stress due to twisting. As a result, the bubbles are further finely divided into fine bubbles.

<Description of operation>
Operation | movement of the evaporative gas reliquefaction apparatus 1 comprised as mentioned above is demonstrated.
The low-temperature liquid 5 (for example, LNG) in the storage tank 3 is sent out by the primary pump 9 and introduced into the fine bubble generator 11. The evaporative gas is boosted to an intermediate pressure by the evaporative gas compressor 7 and then introduced into the fine bubble generator 11. The low-temperature liquid introduced into the fine bubble generator 11 turns into a swirl flow, generates the vapor column 125 of the evaporated gas as described above, and the evaporated gas becomes a fine bubble. By forming the fine bubbles, the contact interface area (heat transfer area) between the evaporation gas and the low-temperature liquid becomes very large, and the evaporation gas that has become the fine bubbles is efficiently cooled and re-liquefied by the low-temperature liquid. Moreover, since the evaporative gas has a lower rising speed in the low-temperature liquid due to the formation of fine bubbles, the contact time between the fine bubbles and the low-temperature liquid becomes longer, and the bubbles gather and coalesce above the low-temperature liquid. Thus, it is possible to prevent the gas and liquid from being separated into two phases. This also makes it possible to liquefy the evaporated gas with certainty.
The bubble diameter and the contact area are in an inversely proportional relationship, and the relationship between the bubble diameter and the rising speed of the bubbles in the liquid is approximately proportional when the bubble diameter is 1 mm or less. For this reason, for example, a fine bubble having a diameter of 100 μm (0.1 mm) has a contact area of 10 times or more and an ascending speed of 1/10 or less as compared with a bubble having a diameter of 1 mm or more generated by a general method. Therefore, it is possible to obtain an overwhelmingly superior reliquefaction performance by reducing the bubble diameter in the reliquefaction of evaporative gas, which is the object of the present invention.

  The evaporated gas that has become fine bubbles is reliquefied into a low-temperature liquid in the fine bubble generator 11 and in a pipe connecting the fine bubble generator 11 and the secondary pump 13. The low-temperature liquid containing the re-liquefied evaporative gas is pressurized by the secondary pump 13, vaporized by the vaporizer 15, and sent to the demand side as city gas.

In the present embodiment, since the evaporation gas is made into fine bubbles and blown into the low-temperature liquid using the fine bubble generator 11, the evaporation gas can be effectively re-liquefied into the low-temperature liquid. It is possible to prevent the evaporative gas from flowing into the pump 13 as a gas and causing a failure in the secondary pump 13.
Further, since the evaporation gas is made into fine bubbles, the re-liquefaction of the evaporation gas is efficiently completed in a short time. Since the distance from the fine bubble generator 11 to the secondary pump 13 can be set short, there is an effect that the degree of freedom of equipment layout is improved and the installation area can be reduced.

  The evaporative gas compressed to an intermediate pressure by the evaporative gas compressor 7 and the low-temperature liquid pressurized by the secondary pump 13 may be heat-exchanged using a heat exchanger. By doing in this way, evaporative gas is cooled and reliquefaction is accelerated | stimulated and vaporization in the vaporizer 15 is accelerated | stimulated by heating a low temperature liquid.

[Embodiment 2]
A second embodiment of the present invention will be described with reference to FIGS.
In FIG. 3, the same parts as those in FIG. The evaporative gas reliquefaction device 16 used in the evaporative gas reliquefaction method of the present embodiment is an evaporative gas that detects the flow rate of the evaporative gas sent to the evaporative gas compressor 7 in addition to the one shown in the first embodiment. The flow rate detector 17, the low temperature liquid flow rate detector 19 for detecting the flow rate of the low temperature liquid delivered from the storage tank 3, the detection signals of the low temperature liquid flow rate detector 19 and the evaporative gas flow rate detector 17 are input, and these And a control device 21 for controlling the pressure for increasing the pressure of the evaporative gas by the evaporative gas compressor 7 based on the detection signal and the pressure for increasing the temperature of the low temperature liquid by the primary pump 9 (corresponding to the “delivery pump” of the present invention). It is a thing.

FIG. 4 is an explanatory diagram of a control method of the evaporative gas compressor 7 by the control device 21, and is a graph showing an example of the relationship between the pressure of the evaporative gas compressor 7 and the primary pump 9 and the ratio of the evaporative gas. is there. The vertical axis indicates the discharge pressure (MPa) of the evaporative gas compressor 7 and the primary pump 9, and the horizontal axis indicates the mass flow ratio (evaporated gas / low temperature liquid) between the evaporated gas and the low temperature liquid.
As shown in FIG. 4, the control device 21 controls the evaporative gas compressor 7 and the primary pump 9 to increase the discharge pressure as the evaporative gas ratio increases.

In this way, by controlling the pressure for boosting the evaporation gas and the low-temperature liquid according to the mass flow rate ratio between the evaporation gas and the low-temperature liquid, the following effects can be obtained.
When the ratio of the evaporative gas is small, it is possible to sufficiently receive the cold heat from the low-temperature liquid, so that the evaporative gas can be reliably liquefied even if the pressure to be increased is lowered. By reducing the pressure to be increased, the power consumption of the evaporative gas compressor 7 can be reduced.
On the other hand, when the proportion of the evaporative gas is large, it is not possible to sufficiently receive the cold heat from the low-temperature liquid. Therefore, the vapor can be reliquefied with a small amount of cold heat by increasing the vapor pressure accordingly, and the proportion of the vaporized gas is large. Even in cases, it can be reliquefied reliably.

As described above, according to the present embodiment, it is possible to reliably reliquefy the evaporative gas while suppressing the evaporative gas compressor 7 to the minimum power consumption.
In addition, when controlling the discharge pressure of the evaporative gas compressor 7 and the primary pump 9, it goes without saying that the pressure increase width of the secondary pump is controlled so that the discharge pressure of the secondary pump 13 becomes the operation pressure for sending out as city gas.

  As a method for controlling the pressure for boosting the evaporative gas and the low temperature liquid according to the mass flow rate ratio between the evaporative gas and the low temperature liquid, the evaporative gas compressor 7 is a multistage multistage evaporative gas compressor, and the primary pump 9 is a multistage multistage booster pump, and the control device 21 determines the number of compression stages of the multistage evaporative gas compressor and the number of booster stages of the multistage booster pump based on the detection signals of the low-temperature liquid flow rate detector 19 and the evaporative gas flow rate detector 17. What is necessary is just to control.

DESCRIPTION OF SYMBOLS 1 Evaporative gas reliquefaction apparatus 3 Storage tank 5 Low temperature liquid 7 Evaporative gas compressor 9 Primary pump 11 Fine bubble generator 13 Secondary pump 15 Evaporator 16 Evaporative gas reliquefaction device 17 Evaporative gas flow detector 19 Low temperature liquid flow detector 21 Control Device 110 Microbubble generator 113 Mixer 115 Bottom surface portion 117 Gas introduction tube 119 Side surface portion 121 Liquid introduction tube 123 Discharge tube 125 Air column

Claims (2)

The evaporative gas generated from the low temperature liquid stored in the storage tank is compressed by an evaporative gas compressor and introduced into the fine bubble generator, and the low temperature liquid in the storage tank is sent out by a delivery pump to the fine bubble generator. An evaporative gas re-liquefaction method for introducing and re-liquefying the evaporative gas by mixing the evaporative gas and the low-temperature liquid in the fine bubble generator, and boosting the low-temperature liquid containing the re-liquefied evaporative gas by a secondary pump Because
The evaporative gas introduced into the microbubble generator becomes microbubbles, and the evaporative gas that has become microbubbles is generated by the cryogenic liquid in the microbubble generator and in the pipe connecting the microbubble generator and the secondary pump. An evaporative gas reliquefaction method characterized by being cooled and reliquefied.   The pressure for boosting the evaporative gas and the cryogenic liquid is controlled according to the mass flow rate ratio between the evaporative gas delivered to the evaporative gas compressor and the cryogenic liquid delivered from the storage tank. The evaporative gas reliquefaction method according to claim 1.

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